Yep, I am getting this too, both on Mint 17 and 18 (both using Cinnamon).
I am getting this too on opensuse tumbleweed with kde 5.12.1
For what it's worth, this isn't happening for me on Kubuntu 17.10, with KDE Plasma 5.10.5. Shutdown happens almost instantly, faster than in Windows most of the time.
EDIT: Oh, I should mention that I originally installed Steam using Discover, KDE's built-in app store thing. That might make my setup slightly different from other people in this thread, but I wouldn't know how exactly.
I am still having this issue on Linux Mint 19 Cinnamon.
I can confirm that not only does this problem still persist on Linux Mint 19.1 but also on Arch Linux with the Cinnamon DE.
Any other DE, like XFCE, KDE, Gnome, and Deepin don't have this problem. I have tested these DE's on Arch, and have specifically tested stock Ubuntu 18.10, Ubuntu 19.04, Kubuntu 18.10, Xubuntu 18.10, KDE Neon, Deepin OS, and Pop! OS 19.04 without this issue.
On Linux Mint/Cinnamon DE, it takes exactly 30 seconds to shutdown/logout from when I click the button.
Your system information
(also tested on kernel 4.4.0-112, also Ubuntu Kernel provided by Linux Mint Update Manager)
Please describe your issue in as much detail as possible:
I had the issue described here, which was very recently fixed on Steam Beta:
https://github.com/ValveSoftware/steam-for-linux/issues/3301
Enabling Beta resolved the original issue, but now shutdown takes an unreasonably longer time to happen on Linux Mint 18.3, as evidenced on these coments to the original issue:
Before switching from normal Steam app to the latest beta I have measured shutdown time as this:
Now, on current beta:
New issue was opened because the sum of the time needed to close Steam and then Linux Mint shutdown before the fix was much smaller (even with human interaction) than now when both actions happen in a direct sequence with no other human interaction after requesting shutdown:
Of note: so far, only Linux Mint Cinnamon 18.3 reported such long delays. Other distros affected by original issue seem unaffected and even a user of Linux Mint KDE 18.2 noticed longer times but not comparable to mine and @Zerophase as evidenced here:
https://github.com/ValveSoftware/steam-for-linux/issues/3301#issuecomment-362939622
Steps for reproducing this issue: