Perhaps this report should be merged with https://github.com/ValveSoftware/steam-for-linux/issues/12005
I did a cursory search and missed this issue reported just a few weeks ago.

I had the same single download source issue, I believe it is been going on for a while, but perhaps on and off, which would cause very slow download speeds.

I only had a single download job which was cache2-den-iwst.steamcontent.com (same as you)
However in my case, I can say that it was only happening when on dual stack with ipv6, although my ipv6 connectivity is excellent and native with my ISP (I do regular speed tests both tcp and udp and bidir), and I also have ipv6 since over 10 years and never had that kind of problem with steam. I confirmed the problem showing only on ipv6 by disabling ipv6 and switching back and forth several times.

What I did temporarily to solve the problem is also to block connections to cache2-den-iwst.steamcontent.com in my dns cache server. It first caused steam to still want to download from that single source but at 0 bps, but then finally it "decided" to connect to others sources, and then voilà, my speed was instantly back at full speed like when I use only ipv4.

This solution is not ideal, and probably not guaranteed to always work. There is also still a 1 minute delay before steam decides to switch and use other sources in my case (so 1 minute before a download starts at all).

I guess if we knew where or how steam gets the download sources list we could edit it directly...

Here's the log of the issue with ipv6:

[2025-06-07 11:16:38] Client version: 1748900261
[2025-06-07 11:16:38] Loaded Steam library folders configuration: /home/user/.local/share/Steam/steamapps/libraryfolders.vdf
[2025-06-07 11:16:38] Loaded 16 apps from install folder "/home/user/.local/share/Steam/steamapps\appmanifest_*.acf".
[2025-06-07 11:16:38] Install folder /mnt/steam_games_subvolume/SteamLibrary not mounted.
[2025-06-07 11:16:38] Loaded 55 apps from install folder "/home/user/steam_games_subvolume/SteamLibrary/steamapps\appmanifest_*.acf".
[2025-06-07 11:16:38] Triggered async write of Steam library folders configuration (3 libraries).
[2025-06-07 11:16:40] AppID x scheduler update : Priority Auto Update, not played for 40426980 seconds, update disabled for 0 seconds
[2025-06-07 11:16:40] AppID x state changed : Update Required,Update Queued,Update Started,
[2025-06-07 11:16:40] AppID x state changed : Update Required,Update Queued,Update Running,Update Started,
[2025-06-07 11:16:40] AppID x App update changed : Running Update,
[2025-06-07 11:16:40] AppID x App update changed : Running Update,Reconfiguring,
[2025-06-07 11:16:40] AppID x App update changed : Running Update,
[2025-06-07 11:16:40] AppID x App update changed : Running Update,Preallocating,
[2025-06-07 11:16:40] AppID x preallocated 0 files (0 MB) 
[2025-06-07 11:16:40] AppID x App update changed : Running Update,
[2025-06-07 11:16:40] AppID x update started : download 17738903424/46933841792, store 0/0, reuse 0/0, delta 0/0, stage 17720904465/46920143663 
[2025-06-07 11:16:40] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:16:40] AppID x App update changed : Running Update,Staging,
[2025-06-07 11:16:40] Got 30 download sources and 0 caching proxies via ContentServerDirectoryService::BYieldingGetServersForSteamPipe (CellID 1 / Launcher 0)
[2025-06-07 11:16:40] Moving to source priority class '6'
[2025-06-07 11:16:40] Created download interface of type 'SteamCache' (7) to host cache2-den-iwst.steamcontent.com (cache2-den-iwst.steamcontent.com)
[2025-06-07 11:16:40] Downloading 27878 chunks from depot 257426
[2025-06-07 11:16:40] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:16:40] HTTP/2 available
[2025-06-07 11:16:40] Created dual-stack http client
[2025-06-07 11:16:53] Increasing target number of download connections to 16 (rate was 0.000, now 53.247)
[2025-06-07 11:17:05] Increasing target number of download connections to 17 (rate was 53.247, now 78.784)
[2025-06-07 11:17:53] Current download rate: 79.780 Mbps
[2025-06-07 11:18:04] AppID x update canceled : Disabled (Suspended)
[2025-06-07 11:18:04] AppID x App update changed : Running Update,Downloading,Staging,Stopping,
[2025-06-07 11:18:04] AppID x App update changed : Running Update,Stopping,
[2025-06-07 11:18:04] AppID x App update changed : None
[2025-06-07 11:18:04] AppID x state changed : Update Required,Update Queued,Update Started, (Suspended)
[2025-06-07 11:18:04] AppID x scheduler finished : staying in schedule (result Suspended, state 0x40a) 
[2025-06-07 11:18:06] HTTPS (SteamCache,441) - cache2-den-iwst.steamcontent.com ([2607:dc0::5]:443 / [2607:dc0::5]:443, host: cache2-den-iwst.steamcontent.com): Closing connection

As you can see, steam even seems to be saying is has 16 download source and increasing, but I can see it is not the case (with iftop or tcpdump for instance).

Here's a log with ipv4 only where everything was working:

[2025-06-07 11:12:31] Client version: 1748900261
[2025-06-07 11:12:31] Loaded Steam library folders configuration: /home/user/.local/share/Steam/steamapps/libraryfolders.vdf
[2025-06-07 11:12:31] Loaded 16 apps from install folder "/home/user/.local/share/Steam/steamapps\appmanifest_*.acf".
[2025-06-07 11:12:31] Install folder /mnt/steam_games_subvolume/SteamLibrary not mounted.
[2025-06-07 11:12:31] Loaded 55 apps from install folder "/home/user/steam_games_subvolume/SteamLibrary/steamapps\appmanifest_*.acf".
[2025-06-07 11:12:31] Triggered async write of Steam library folders configuration (3 libraries).
[2025-06-07 11:12:33] AppID x scheduler update : Priority Auto Update, not played for 40426733 seconds, update disabled for 0 seconds
[2025-06-07 11:12:33] AppID x state changed : Update Required,Update Queued,Update Started,
[2025-06-07 11:12:33] AppID x state changed : Update Required,Update Queued,Update Running,Update Started,
[2025-06-07 11:12:33] AppID x App update changed : Running Update,
[2025-06-07 11:12:33] AppID x App update changed : Running Update,Reconfiguring,
[2025-06-07 11:12:33] AppID x App update changed : Running Update,
[2025-06-07 11:12:33] AppID x App update changed : Running Update,Preallocating,
[2025-06-07 11:12:33] AppID x preallocated 0 files (0 MB) 
[2025-06-07 11:12:33] AppID x App update changed : Running Update,
[2025-06-07 11:12:33] AppID x update started : download 16872186384/46933841792, store 0/0, reuse 0/0, delta 0/0, stage 16859252619/46920143663 
[2025-06-07 11:12:33] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:12:33] AppID x App update changed : Running Update,Staging,
[2025-06-07 11:12:33] Got 30 download sources and 0 caching proxies via ContentServerDirectoryService::BYieldingGetServersForSteamPipe (CellID 1 / Launcher 0)
[2025-06-07 11:12:33] Moving to source priority class '3'
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache4-ord1.steamcontent.com (cache4-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache10-ord1.steamcontent.com (cache10-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache15-ord1.steamcontent.com (cache15-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache5-ord1.steamcontent.com (cache5-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache14-ord1.steamcontent.com (cache14-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache3-ord1.steamcontent.com (cache3-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache6-ord1.steamcontent.com (cache6-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache13-ord1.steamcontent.com (cache13-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache12-ord1.steamcontent.com (cache12-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache8-sea1.steamcontent.com (cache8-sea1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache1-sea1.steamcontent.com (cache1-sea1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache10-sea1.steamcontent.com (cache10-sea1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache16-ord1.steamcontent.com (cache16-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache11-ord1.steamcontent.com (cache11-ord1.steamcontent.com)
[2025-06-07 11:12:33] Created download interface of type 'SteamCache' (7) to host cache7-ord1.steamcontent.com (cache7-ord1.steamcontent.com)
[2025-06-07 11:12:33] Downloading 28700 chunks from depot 257426
[2025-06-07 11:12:33] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:12:33] HTTP/2 available
[2025-06-07 11:12:33] Created ipv4-only http client
[2025-06-07 11:12:46] Increasing target number of download connections to 16 (rate was 0.000, now 483.282)
[2025-06-07 11:12:46] Created download interface of type 'SteamCache' (7) to host cache6-sea1.steamcontent.com (cache6-sea1.steamcontent.com)
[2025-06-07 11:12:48] AppID x update canceled : Disabled (Suspended)
[2025-06-07 11:12:48] AppID x App update changed : Running Update,Downloading,Staging,Stopping,
[2025-06-07 11:12:48] AppID x App update changed : Running Update,Stopping,
[2025-06-07 11:12:48] AppID x App update changed : None
[2025-06-07 11:12:48] AppID x state changed : Update Required,Update Queued,Update Started, (Suspended)
[2025-06-07 11:12:48] AppID x scheduler finished : staying in schedule (result Suspended, state 0x40a) 
[2025-06-07 11:12:50] HTTPS (SteamCache,286) - cache6-sea1.steamcontent.com (205.196.6.167:443 / 205.196.6.167:443, host: cache6-sea1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,272) - cache7-ord1.steamcontent.com (162.254.193.4:443 / 162.254.193.4:443, host: cache7-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,435) - cache11-ord1.steamcontent.com (162.254.193.6:443 / 162.254.193.6:443, host: cache11-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,513) - cache16-ord1.steamcontent.com (162.254.193.44:443 / 162.254.193.44:443, host: cache16-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,465) - cache10-sea1.steamcontent.com (205.196.6.173:443 / 205.196.6.173:443, host: cache10-sea1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,1) - cache1-sea1.steamcontent.com (205.196.6.165:443 / 205.196.6.165:443, host: cache1-sea1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,240) - cache8-sea1.steamcontent.com (205.196.6.170:443 / 205.196.6.170:443, host: cache8-sea1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,436) - cache12-ord1.steamcontent.com (162.254.193.42:443 / 162.254.193.42:443, host: cache12-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,437) - cache13-ord1.steamcontent.com (162.254.193.7:443 / 162.254.193.7:443, host: cache13-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,118) - cache6-ord1.steamcontent.com (162.254.193.38:443 / 162.254.193.38:443, host: cache6-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,473) - cache3-ord1.steamcontent.com (162.254.193.14:443 / 162.254.193.14:443, host: cache3-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,438) - cache14-ord1.steamcontent.com (162.254.193.43:443 / 162.254.193.43:443, host: cache14-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,117) - cache5-ord1.steamcontent.com (162.254.193.13:443 / 162.254.193.13:443, host: cache5-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,512) - cache15-ord1.steamcontent.com (162.254.193.8:443 / 162.254.193.8:443, host: cache15-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,231) - cache10-ord1.steamcontent.com (162.254.193.40:443 / 162.254.193.40:443, host: cache10-ord1.steamcontent.com): Closing connection
[2025-06-07 11:12:50] HTTPS (SteamCache,474) - cache4-ord1.steamcontent.com (162.254.193.36:443 / 162.254.193.36:443, host: cache4-ord1.steamcontent.com): Closing connection

And here's the log when I blacklisted cache2-den-iwst.steamcontent.com:

[2025-06-07 11:35:40] Client version: 1748900261
[2025-06-07 11:35:40] Loaded Steam library folders configuration: /home/user/.local/share/Steam/steamapps/libraryfolders.vdf
[2025-06-07 11:35:40] Loaded 16 apps from install folder "/home/user/.local/share/Steam/steamapps\appmanifest_*.acf".
[2025-06-07 11:35:40] Install folder /mnt/steam_games_subvolume/SteamLibrary not mounted.
[2025-06-07 11:35:40] Loaded 55 apps from install folder "/home/user/steam_games_subvolume/SteamLibrary/steamapps\appmanifest_*.acf".
[2025-06-07 11:35:40] Triggered async write of Steam library folders configuration (3 libraries).
[2025-06-07 11:35:42] AppID x scheduler update : Priority Auto Update, not played for 40428122 seconds, update disabled for 0 seconds
[2025-06-07 11:35:42] AppID x state changed : Update Required,Update Queued,Update Started,
[2025-06-07 11:35:42] AppID x state changed : Update Required,Update Queued,Update Running,Update Started,
[2025-06-07 11:35:42] AppID x App update changed : Running Update,
[2025-06-07 11:35:42] AppID x App update changed : Running Update,Reconfiguring,
[2025-06-07 11:35:42] AppID x App update changed : Running Update,
[2025-06-07 11:35:42] AppID x App update changed : Running Update,Preallocating,
[2025-06-07 11:35:42] AppID x preallocated 0 files (0 MB) 
[2025-06-07 11:35:42] AppID x App update changed : Running Update,
[2025-06-07 11:35:42] AppID x update started : download 20114699984/46933841792, store 0/0, reuse 0/0, delta 0/0, stage 20085962837/46920143663 
[2025-06-07 11:35:42] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:35:42] AppID x App update changed : Running Update,Staging,
[2025-06-07 11:35:42] Got 30 download sources and 0 caching proxies via ContentServerDirectoryService::BYieldingGetServersForSteamPipe (CellID 20 / Launcher 0)
[2025-06-07 11:35:42] Moving to source priority class '6'
[2025-06-07 11:35:42] Created download interface of type 'SteamCache' (7) to host cache2-den-iwst.steamcontent.com (cache2-den-iwst.steamcontent.com)
[2025-06-07 11:35:42] Downloading 25614 chunks from depot 257426
[2025-06-07 11:35:42] AppID x App update changed : Running Update,Downloading,Staging,
[2025-06-07 11:35:42] HTTP/2 available
[2025-06-07 11:35:42] Created dual-stack http client
[2025-06-07 11:36:43] HTTPS (SteamCache,441) - cache2-den-iwst.steamcontent.com (0.0.0.0:443 / 0.0.0.0:443, host: cache2-den-iwst.steamcontent.com): Received 0 (Invalid) HTTP response for depot 257426
[2025-06-07 11:36:43] HTTPS (SteamCache,441) - cache2-den-iwst.steamcontent.com (0.0.0.0:443 / 0.0.0.0:443, host: cache2-den-iwst.steamcontent.com): Closing connection
[2025-06-07 11:36:43] stats: (SteamCache, 441) cache2-den-iwst.steamcontent.com: 0 Bytes, 0 sec (0.00 Mbps). 0 Hits / 0 Misses (0 %, 0 % bytes)
[2025-06-07 11:36:43] Moving to source priority class '3'
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache4-ord1.steamcontent.com (cache4-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache15-ord1.steamcontent.com (cache15-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache10-ord1.steamcontent.com (cache10-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache10-sea1.steamcontent.com (cache10-sea1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache8-sea1.steamcontent.com (cache8-sea1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache1-sea1.steamcontent.com (cache1-sea1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache5-ord1.steamcontent.com (cache5-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache14-ord1.steamcontent.com (cache14-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache3-ord1.steamcontent.com (cache3-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache6-ord1.steamcontent.com (cache6-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache13-ord1.steamcontent.com (cache13-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache12-ord1.steamcontent.com (cache12-ord1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache6-sea1.steamcontent.com (cache6-sea1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache5-sea1.steamcontent.com (cache5-sea1.steamcontent.com)
[2025-06-07 11:36:43] Created download interface of type 'SteamCache' (7) to host cache7-ord1.steamcontent.com (cache7-ord1.steamcontent.com)
[2025-06-07 11:36:47] Detected write gap 65 MB in file "..."
[2025-06-07 11:36:56] Increasing target number of download connections to 16 (rate was 0.000, now 490.520)
[2025-06-07 11:36:56] Current download rate: 490.520 Mbps
[2025-06-07 11:36:56] Created download interface of type 'SteamCache' (7) to host cache19-iad1.steamcontent.com (cache19-iad1.steamcontent.com)
[2025-06-07 11:37:25] Detected write gap 70 MB in file "..."
[2025-06-07 11:37:25] Detected write gap 130 MB in file "..."
[2025-06-07 11:37:27] Detected write gap 94 MB in file "..."
[2025-06-07 11:37:28] Detected write gap 67 MB in file "..."
[2025-06-07 11:37:38] Detected write gap 100 MB in file "..."
[2025-06-07 11:37:40] Detected write gap 101 MB in file "..."
[2025-06-07 11:37:51] Detected write gap 71 MB in file "..."
[2025-06-07 11:37:57] Current download rate: 508.292 Mbps
[2025-06-07 11:37:58] Detected write gap 67 MB in file "..."
[2025-06-07 11:38:17] Detected write gap 109 MB in file "..."
[2025-06-07 11:38:22] Detected write gap 65 MB in file "..."
[2025-06-07 11:38:28] Detected write gap 80 MB in file "..."
[2025-06-07 11:38:29] Detected write gap 77 MB in file "..."
[2025-06-07 11:38:43] Detected write gap 70 MB in file "..."
[2025-06-07 11:38:50] Detected write gap 73 MB in file "..."
[2025-06-07 11:38:52] Detected write gap 71 MB in file "..."
[2025-06-07 11:38:54] Detected write gap 99 MB in file "..."
[2025-06-07 11:38:58] Current download rate: 505.500 Mbps
[2025-06-07 11:38:59] AppID x update canceled : Disabled (Suspended)
[2025-06-07 11:38:59] AppID x App update changed : Running Update,Downloading,Staging,Stopping,
[2025-06-07 11:38:59] HTTPS (SteamCache,231) - cache10-ord1.steamcontent.com ([2602:801:f007:101::a2fe:c128]:443 / [2602:801:f007:101::a2fe:c128]:443, host: cache10-ord1.steamcontent.com): Closing connection
[2025-06-07 11:38:59] HTTPS (SteamCache,1) - cache1-sea1.steamcontent.com ([2602:801:f00b:100::cdc4:6a5]:443 / [2602:801:f00b:100::cdc4:6a5]:443, host: cache1-sea1.steamcontent.com): Closing connection
[2025-06-07 11:38:59] AppID x App update changed : Running Update,Stopping,
[2025-06-07 11:38:59] AppID x App update changed : None
[2025-06-07 11:38:59] AppID x state changed : Update Required,Update Queued,Update Started, (Suspended)
[2025-06-07 11:38:59] AppID x scheduler finished : staying in schedule (result Suspended, state 0x40a) 
[2025-06-07 11:39:01] HTTPS (SteamCache,502) - cache19-iad1.steamcontent.com ([2602:801:f002:102::a2fe:c02a]:443 / [2602:801:f002:102::a2fe:c02a]:443, host: cache19-iad1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,272) - cache7-ord1.steamcontent.com ([2602:801:f007:100::a2fe:c104]:443 / [2602:801:f007:100::a2fe:c104]:443, host: cache7-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,285) - cache5-sea1.steamcontent.com ([2602:801:f00b:100::cdc4:6a6]:443 / [2602:801:f00b:100::cdc4:6a6]:443, host: cache5-sea1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,286) - cache6-sea1.steamcontent.com ([2602:801:f00b:100::cdc4:6a7]:443 / [2602:801:f00b:100::cdc4:6a7]:443, host: cache6-sea1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,436) - cache12-ord1.steamcontent.com ([2602:801:f007:101::a2fe:c12a]:443 / [2602:801:f007:101::a2fe:c12a]:443, host: cache12-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,437) - cache13-ord1.steamcontent.com ([2602:801:f007:100::a2fe:c107]:443 / [2602:801:f007:100::a2fe:c107]:443, host: cache13-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,118) - cache6-ord1.steamcontent.com ([2602:801:f007:101::a2fe:c126]:443 / [2602:801:f007:101::a2fe:c126]:443, host: cache6-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,473) - cache3-ord1.steamcontent.com ([2602:801:f007:100::a2fe:c10e]:443 / [2602:801:f007:100::a2fe:c10e]:443, host: cache3-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,438) - cache14-ord1.steamcontent.com ([2602:801:f007:101::a2fe:c12b]:443 / [2602:801:f007:101::a2fe:c12b]:443, host: cache14-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,117) - cache5-ord1.steamcontent.com ([2602:801:f007:100::a2fe:c10d]:443 / [2602:801:f007:100::a2fe:c10d]:443, host: cache5-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,240) - cache8-sea1.steamcontent.com ([2602:801:f00b:100::cdc4:6aa]:443 / [2602:801:f00b:100::cdc4:6aa]:443, host: cache8-sea1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,465) - cache10-sea1.steamcontent.com ([2602:801:f00b:100::cdc4:6ad]:443 / [2602:801:f00b:100::cdc4:6ad]:443, host: cache10-sea1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,512) - cache15-ord1.steamcontent.com ([2602:801:f007:100::a2fe:c108]:443 / [2602:801:f007:100::a2fe:c108]:443, host: cache15-ord1.steamcontent.com): Closing connection
[2025-06-07 11:39:01] HTTPS (SteamCache,474) - cache4-ord1.steamcontent.com ([2602:801:f007:101::a2fe:c124]:443 / [2602:801:f007:101::a2fe:c124]:443, host: cache4-ord1.steamcontent.com): Closing connection

I think the "Moving to source priority class '6'" vs "Moving to source priority class '3'" is what makes the difference here.
It seems whenever it is priority class 6, it is slow, perhaps this is some kind of list of sources of different "priority class" (which I don't really know what it means or is used for), and there is only one source?
When on ipv4, it seems to always start with source priority class '3', which may explain why it is fast.

• Distribution: PopOS
• Opted into Steam client beta?: Yes (version in log above)
• Have you checked for system updates?: Yes
• Steam Logs: above
• GPU: AMD RX 6750

~Also just to add to this, on my side I still need to add @nClientDownloadEnableHTTP2PlatformLinux 0 to my steam_dev.cfg to get good download speeds, otherwise regardless of the source blacklisting I would get bad download speeds with both ipv4 and ipv6. I think this issue with single download source is different.~

I didn't realize that it was now turned off by default. I previously tested that it was still needed by manually setting it to 0 or 1, but didn't realize there was no need to set it anymore.

However in my case, I can say that it was only happening when on dual stack with ipv6

I just tried disabling IPv6 and I do not see the issue occurring as well.

I think the "Moving to source priority class '6'" vs "Moving to source priority class '3'" is what makes the difference here.

I am also seeing this same behavior.

I did a little deeper digging into this.

The Steam client appears to query https://api.steampowered.com/IContentServerDirectoryService/GetServersForSteamPipe/v1/?cell_id=REPLACE_ME&max_servers=30&launcher_type=0&ipv6_public=REPLACE_ME for a list of servers to connect to.

It shows one server with priority_class 6, along with others with priority classes 3, 2, and 1, and the 6 one is the one that I had to block.

The ipv6_public value is the ipv6 of the device that Steam is on, cell_id is the cell ID that is shown in the logs. Though it doesn't appear to matter what it is set to, the results only change when the ipv6_public value is changed. Without the ipv6_public value it doesn't return the priority_class 6 server.

Note that while I was checking this the server would sometimes not appear in the list, waiting and refreshing would show it again.

Nice find!
I hope this get fixed, this seems like a major performance bottleneck if you are using ipv6.

Note that while I was checking this the server would sometimes not appear in the list, waiting and refreshing would show it again.

It makes sense, sometimes I would not have slow download issues.

So far the workaround we have is to block the problematic server by hand, but this could change, so not a good solution.

I guess one could create a reverse https proxy and filter the priority class 6 (like a simple node.js app), with self signed certificate and adding it to the trusted root certificates and rerouting the dns...

Another strange workaround I found is, if you select a European Download region (Germany Frankfurt for example) it does not seem they return class 6. So in my case, this resulted in very fast download speeds as well, without having to block any server. I never thought a solution to make my internet download faster would be to download from far overseas servers...

I guess one could create a reverse https proxy and filter the priority class 6 (like a simple node.js app), with self signed certificate and adding it to the trusted root certificates and rerouting the dns...

It looks like it actually communicates mostly over web sockets, I had thought it was querying that API endpoint due to the DNS query for api.steampowered.com just before the initial connection. I originally did a search for the GetServersForSteamPipe and came across that endpoint. It appears to only return the Denver cache and no other servers too.

I am not sure what the difference is between Windows and Linux and this is starting to be out of my area of understanding.

The best I can think of is that certain IPv6 ranges seem to be matched with incorrect location data that makes it choose the Denver or other single server, and for some reason the Linux client doesn't grab any other servers.

It looks like it actually communicates mostly over web sockets, I had thought it was querying that API endpoint due to the DNS query for api.steampowered.com just before the initial connection. I originally did a search for the GetServersForSteamPipe and came across that endpoint. It appears to only return the Denver cache and no other servers too.

Yes, I also realized that, yesterday, after I actually made that little node.js proxy. It did work well, and captured and proxied all normal API calls steam was doing to api.steampowered.com, but I could not see the GetServersForSteamPipe call being made, so I guessed that it was doing it in some other ways like over websocket...

The best I can think of is that certain IPv6 ranges seem to be matched with incorrect location data that makes it choose the Denver or other single server, and for some reason the Linux client doesn't grab any other servers.

I did some tests with different ipv6 prefixes of different length, even of the same geographical area, and yes I can see the response changes depending on the ipv6 and would not have that slow server and speed issue. I am not too sure why they have to pass our ipv6, they should content with using the cellid specified only (that would be easier to troubleshoot for everyone including valve/steam), there is already a lot of regions to choose from in the steam client.

I hope it is only a bug or a mistake in the implementation, and not some kind of traffic shaping attempt, because I think this would be unacceptable. If that would be the case, the steam app should at least tell users "Your downloads are slower due to ..."

can confirm I have this same issue
worked around it by blocking the domains that steam downloading was stuck on by editing /etc/hosts

Yes, I can confirm I have exact same bug (ZorinOS18). I have also noticed that it does not matter which server I hop to, it is always their respective cache1 and cache2 addresses that are being used only (as Download jobs), causing slow speeds.

As soon as I block cache1 and cache2 in etc/hosts, Steam will have multiple jobs and max my broadband. It is actually not necessary to block both, you can either block cache1 or cache2, it will still work fine.

I am also experiencing this.

System Information
OS: Fedora Linux 43 (KDE Plasma Desktop Edition) x86_64
Kernel: Linux 6.18.8-200.fc43.x86_64
DE: KDE Plasma 6.5.5
WM: KWin (Wayland)

Steam Beta Branch:  Stable Client
Steam Version:  1769025840
Steam Client Build Date:  Wed, Jan 21 11:12 AM UTC -08:00
Steam Web Build Date:  Wed, Jan 21 1:41 PM UTC -08:00
Steam API Version:  SteamClient023

Before blocking cache2-den-iwst.steamcontent.com

I was getting ~100 Mbps on that 1 connection.

After blocking in /etc/hosts

I was able to get 16 simultaneous connections, and I reached peak 1.6 Gbps (still about 1 Gbps lower than my 2.5 Gbps link from my ISP, but I'll take a 10x improvement)

Can confirm the same issue and fix on Arch Linux (Omarchy). Download region set to Adelaide, Australia.

Steam was pinning to a single CDN server (cache1-syd-vocs.steamcontent.com) and capping at ~150 Mbps on a 1000 Mbps connection. Blocked the server via as other users above have done /etc/hosts:

127.0.0.1 cache1-syd-vocs.steamcontent.com

After restarting Steam, there's a brief delay while the client fails over, then it distributes across multiple servers — Melbourne, Brisbane, Sydney, Auckland, plus Fastly and Akamai CDNs:

Download sources:
- host cache1-mel-vocs.steamcontent.com, cell 53, SteamCache ([#429](/issue/ValveSoftware/steam-for-linux/429)), 1 connections of 1 max
- host cache1-bne-edgx.steamcontent.com, cell 51, SteamCache ([#208](/issue/ValveSoftware/steam-for-linux/208)), 1 connections of 1 max
- host cache2-bne-edgx.steamcontent.com, cell 51, SteamCache ([#218](/issue/ValveSoftware/steam-for-linux/218)), 1 connections of 1 max
- host cache1-mel-edgx.steamcontent.com, cell 53, SteamCache ([#41](/issue/ValveSoftware/steam-for-linux/41)), 1 connections of 1 max
- host cache2-mel-edgx.steamcontent.com, cell 53, SteamCache ([#42](/issue/ValveSoftware/steam-for-linux/42)), 1 connections of 1 max
- host cache1-akl-edgx.steamcontent.com, cell 22, SteamCache ([#498](/issue/ValveSoftware/steam-for-linux/498)), 1 connections of 1 max
- host cache3-syd1.steamcontent.com, cell 52, SteamCache ([#86](/issue/ValveSoftware/steam-for-linux/86)), 1 connections of 1 max
- host cache7-syd1.steamcontent.com, cell 52, SteamCache ([#554](/issue/ValveSoftware/steam-for-linux/554)), 1 connections of 1 max
- host cache8-syd1.steamcontent.com, cell 52, SteamCache ([#555](/issue/ValveSoftware/steam-for-linux/555)), 1 connections of 1 max
- host cache1-syd1.steamcontent.com, cell 52, SteamCache ([#179](/issue/ValveSoftware/steam-for-linux/179)), 1 connections of 1 max
- host cache6-syd1.steamcontent.com, cell 52, SteamCache ([#94](/issue/ValveSoftware/steam-for-linux/94)), 1 connections of 1 max
- host cache5-syd1.steamcontent.com, cell 52, SteamCache ([#93](/issue/ValveSoftware/steam-for-linux/93)), 1 connections of 1 max
- host cache4-syd1.steamcontent.com, cell 52, SteamCache (#6), 1 connections of 1 max
- host cache2-syd1.steamcontent.com, cell 52, SteamCache ([#85](/issue/ValveSoftware/steam-for-linux/85)), 1 connections of 1 max
- host fastly.cdn.steampipe.steamcontent.com, cell 0, CDN ([#31](/issue/ValveSoftware/steam-for-linux/31)), 1 connections of 1 max
- host steampipe.akamaized.net, cell 0, CDN ([#11](/issue/ValveSoftware/steam-for-linux/11)), 1 connections of 1 max

Result: 150 Mbps → 730+ Mbps. This issue has affected me for over a year. Thanks for the workaround!

Just adding my two cents; I had this issue as well. I could never get above 1Gbps with a 2Gbps link. This fixed it immediately. I now steadily get 1.7/1.8Gbps on downloads.

Editing to add: I do manage to get high speeds now, but for some reason it won't maintain the speed. I'm not entirely sure why but there's a lot of fluctuation.

Chiming in my own 2 cents here, I'm running Kubuntu using the .deb package provided by Valve, and it's the same thing. I had to block a specific server and it immediately went from about 280Mbps to over 700Mbps on average. My network is nowhere near saturated with that, though. I have a 2.5Gb NIC and the ISP to support it. Usually see anywhere from 1.6Gbps to 1.8Gbps on my Windows install.

I have this similar issue of restricted download jobs only having 2 streams that restricts the bandwidth. When I exclude 2 stream URLS in DNS, steam will proceed to add more streams. This improves download speed significantly.

I have seen this behavior on Fedora and different Arch variants. Clearly there is something steam can do on their end to improve download speed. In function of how the client works.

I used /etc/hosts to block the slow sources as found in download_sources but Steam either just replaces them with more slow ones or continues to try and force the blocked sources despite them failing. This leads to a situation in which - instead of just one or two concurrent slow download sources and nothing else - I have 1 or two slow download sources while steam continues trying and retrying the slow ones I've already blocked without trying other ones from the pool of potential options

For some reason I am only getting 3 sources plus CDN sources, and the CDN sources are never being used. I have consistent 2.3-2.4 Gbps speeds to iperf3 servers near IAD. No idea why I'm being served just two IAD servers and one ORD server when I have Washington DC set as my Download region. When I block these hosts using /etc/hosts/ the download just never starts and never falls back to the CDN.

Edit: If I block two of the SteamCache hosts the CDNs start getting used and I can max my connection.

Download sources:
- host cache4-iad1.steamcontent.com, vhost cache4-iad1.steamcontent.com, cell 63, SteamCache ([#467](/issue/ValveSoftware/steam-for-linux/467)), 1 connections of 1 max
- host cache3-iad1.steamcontent.com, vhost cache3-iad1.steamcontent.com, cell 63, SteamCache ([#466](/issue/ValveSoftware/steam-for-linux/466)), 1 connections of 1 max
- host cache16-ord1.steamcontent.com, vhost cache16-ord1.steamcontent.com, cell 1, SteamCache ([#513](/issue/ValveSoftware/steam-for-linux/513)), 1 connections of 1 max
- host alibaba.cdn.steampipe.steamcontent.com, vhost alibaba.cdn.steampipe.steamcontent.com, cell 0, CDN ([#33](/issue/ValveSoftware/steam-for-linux/33)), 0 connections of 1 max
- host steampipe.akamaized.net, vhost steampipe.akamaized.net, cell 0, CDN ([#11](/issue/ValveSoftware/steam-for-linux/11)), 0 connections of 1 max
- host fastly.cdn.steampipe.steamcontent.com, vhost fastly.cdn.steampipe.steamcontent.com, cell 0, CDN ([#31](/issue/ValveSoftware/steam-for-linux/31)), 0 connections of 1 max
- host google2.cdn.steampipe.steamcontent.com, vhost google2.cdn.steampipe.steamcontent.com, cell 0, CDN ([#35](/issue/ValveSoftware/steam-for-linux/35)), 0 connections of 1 max
Download jobs:
- type SteamCache, name cache4-iad1.steamcontent.com, iteration 1, chunks: index 12671, 86 outstanding (75.36 MB), 11902 deferred, 769 requested, 683 succeeded, 0 failed. 497.3 Mbps 
- type SteamCache, name cache3-iad1.steamcontent.com, iteration 1, chunks: index 8297, 0 outstanding (0 bytes), 8035 deferred, 262 requested, 262 succeeded, 0 failed. 153.1 Mbps 
- type SteamCache, name cache16-ord1.steamcontent.com, iteration 1, chunks: index 459, 185 outstanding (161.71 MB), 0 deferred, 459 requested, 274 succeeded, 0 failed. 208.1 Mbps ```

Replying to https://github.com/ValveSoftware/steam-for-linux/issues/12082#issuecomment-4520476408

There are other threads that can give you some steam console commands to add more download sources, and to make the client add more connections dynamically.

steam://open/console - in your favorite terminal

@fDownloadRateImprovementToAddAnotherConnection 1.0
@cMaxInitialDownloadSources 15

Replying to https://github.com/ValveSoftware/steam-for-linux/issues/12082#issuecomment-4640709974

Yeah, those commands are everywhere on the internet. They did nothing for me.

👎

👍

@PeerContentClientMaxJobs 20

wonder why this is in the list: `cache16-fra2.steamcontent.com

@vulptex where did you add this config / flag?

@vulptex where did you add this config / flag?

console or config (see first post)

Having this same issue on Fedora Linux 44 (KDE Plasma Desktop Edition) x86_64.

@PeerContentClientMaxJobs 20 doesn't do anything

I have to block 0.0.0.0 cache15-ord1.steamcontent.com in my hosts file, but this is only a temporary fix. Next time I start steam, it just refuses to download anything, so I have to unblock and restart steam, then reblock it.

Can confirm blocking SteamCaches in /etc/hosts resulted in Steam using more than 1 connection. I went from 100mbps to 900mbps and 1 connection to 5. Disabling ipv6 had no effect. It was stuck on a single slow SteamCache connection before but now it's using the CDNs as well. Only seeing this on my Arch Linux machine, my windows machine maxes out my download speed with no tweaks.

/etc/hosts

0.0.0.0 cache1-sea1.steamcontent.com
0.0.0.0 cache13-iad1.steamcontent.com
0.0.0.0 cache15-ord1.steamcontent.com

I also had to do this on my steam deck. Insanely surprised this hasn't been fixed yet.

Reproducing the same single-download-job issue on Linux. Only 2 download jobs (both on the same local CDN, Ezeiza/Argentina) while a 1 Gbps connection was stuck at ~120 Mbps.

Before:

[2026-08-13 18:34:58] Current download rate: 122.519 Mbps
[2026-08-13 18:37:00] Current download rate: 133.781 Mbps
[2026-08-13 18:38:45] Got 6 download sources and 0 caching proxies via ContentServerDirectoryService::BYieldingGetServersForSteamPipe (CellID 116 / Launcher 0)

Only 2 sources were actually used (cache1/cache2-eze-pita.steamcontent.com), both served over native IPv6.

Workaround (same as other reports): block those two hostnames in /etc/hosts (both A and AAAA records), pause, resume. After Steam fell back to the full server directory:

[2026-08-13 18:41:24] (write gaps while downloading)
[2026-08-13 18:42:05] Current download rate: 897.391 Mbps
Got 22 download sources

Speed went from ~130 Mbps to ~897 Mbps (1 Gbps plan) with zero config changes.

The workaround is not a fix — if I hit the same problem again I would have to re-block whichever 2 servers Steam picks. Root cause seems to be Steam committing to too few servers from ContentServerDirectoryService instead of spreading chunks across the available pool, which gets worse over IPv6.

System: Debian GNU/Linux 13 (trixie), kernel 6.12.101+deb13-amd64, AMD Ryzen 5 5600G, 46 GiB RAM, Steam client 1782344391, native IPv6.

Fedora 44 and I'm having the same issue. Using the /etc/hosts workaround to block the hostnames work. Denver server causing me issues (I have my download settings to San Jose since I'm west coast).

Interestingly, this only started happening as soon as I switched my ISP. I went from Optimum cable internet to Vero fiber internet. Different ISP router and Vero uses a CGNAT. Vero doesn't give IPv6 - only IPv4. Not sure if just coincidence or related.

System: Fedora 44. Kernel Version: 7.1.9-200.fc44.x86_64 (64-bit). Ryzen 9 5950X 16-Core Processor. Radeon RX 7900 XTX.

Workaround: Steam Linux stuck on one slow regional cache

Environment

• Arch Linux
• Native Steam client, build 1785799196
• Fast NVMe storage
• Wi-Fi link negotiated above 2 Gbps
• Other network traffic performed normally

Problem

Steam downloads were much slower on Linux than Windows. Changing the configured download region did not reliably select a geographically matching server.

Observed performance:

• cache1-sea1.steamcontent.com: approximately 83-97 Mbps
• cache13-iad1.steamcontent.com: approximately 158-194 Mbps
• Generic multi-CDN fallback: approximately 630-760 Mbps

Steam was selecting one regional SteamCache download job even though several generic CDNs were available but inactive.

Diagnosis

Open the Steam console by opening this URI in a browser:

steam://open/console

While downloading a large game, run:

download_sources

The slow state looked like this:

cache1-sea1.steamcontent.com: 1 connection
Fastly: 0 connections
Alibaba: 0 connections
Akamai: 0 connections
Google: 0 connections

Download jobs:
cache1-sea1.steamcontent.com: 86.6 Mbps

This resembles the following reports:

• ValveSoftware/steam-for-linux#12082
• ValveSoftware/steam-for-linux#13378

Temporary test

The currently selected cache addresses were temporarily made unreachable:

sudo ip -6 route add unreachable 2602:801:f00b:100::cdc4:6a5/128
sudo ip -6 route add unreachable 2602:801:f002:102::a2fe:c024/128

After pausing and resuming the download, Steam abandoned the regional caches and activated multiple CDNs.

Temporary rollback:

sudo ip -6 route del unreachable 2602:801:f00b:100::cdc4:6a5/128
sudo ip -6 route del unreachable 2602:801:f002:102::a2fe:c024/128

These routes disappear after reboot. The IPs are specific to my session and should not be copied blindly.

Persistent workaround

First, back up the hosts file:

sudo cp -a /etc/hosts /etc/hosts.bak

Then edit it:

sudoedit /etc/hosts

I added only the two caches confirmed to be slow on my connection:

# Avoid underperforming Steam regional caches.
127.0.0.1 cache1-sea1.steamcontent.com
::1       cache1-sea1.steamcontent.com
127.0.0.1 cache13-iad1.steamcontent.com
::1       cache13-iad1.steamcontent.com

Flush DNS and restart Steam:

sudo resolvectl flush-caches
steam -shutdown
steam

Result

Afterward, download_sources showed:

• Seattle cache: 0 connections
• IAD cache: 0 connections
• Alibaba: active
• Akamai: active
• Fastly: active
• Google: active

Example rates from one download:

Alibaba: 258.6 Mbps
Akamai:  256.4 Mbps
Fastly:   29.5 Mbps
Google:   119.5 Mbps
Total:   664.0 Mbps

The highest Steam-reported rate observed was approximately 760.8 Mbps.

Optional steam_dev.cfg experiment

I am also testing this minimal configuration:

@fDownloadRateImprovementToAddAnotherConnection 1.0
@cMaxInitialDownloadSources 20

Location:

~/.local/share/Steam/steam_dev.cfg

This encourages Steam to add connections sooner and allows more initial candidate sources. It does not disable HTTP/2, and it is not required for the verified cache-blocking improvement.

I did not use the older HTTP/2 workaround:

@nClientDownloadEnableHTTP2PlatformLinux 0

The associated [Valve issue #10248](https://github.com/ValveSoftware/steam-for-linux/issues/10248) is marked resolved.

Rollback

Remove only the added Steam entries from /etc/hosts, then run:

sudo resolvectl flush-caches
steam -shutdown
steam

Alternatively, if /etc/hosts has not received other changes:

sudo install -o root -g root -m 0644 /etc/hosts.bak /etc/hosts
sudo resolvectl flush-caches

Caveats

• Identify your own slow host with download_sources; do not blindly block mine.
• Cache hostnames and addresses can change.
• Steam may occasionally report "No connection" instead of falling back. If so, revert the block.
• This is a workaround and not an official fix from Valve.

TL;DR

Steam on Linux was choosing one slow regional cache and ignoring faster CDNs. Downloads were stuck
around 86–97 Mbps.

Open steam://open/console, run:

download_sources

Identify the single slow active cache, then map only that hostname to loopback in /etc/hosts:

127.0.0.1 slow-cache-hostname.steamcontent.com
::1 slow-cache-hostname.steamcontent.com

Then run:

sudo resolvectl flush-caches
steam -shutdown
steam

Steam fell back to Alibaba, Akamai, Fastly, and Google simultaneously, increasing downloads to
approximately 664–760 Mbps.

Do not blindly copy my blocked hostnames; identify your own slow cache first. The old HTTP/2 disable
workaround was not required.

That's some incredible investigations @davgar99 , hopefully the steam team can use this!