As discussed here: https://gist.github.com/FikriRNurhidayat/ce18426ad94fff2140538c0adf0e06ec

On multiple Linux distributions (Fedora, Arch, Debian/Ubuntu-based), Steam downloads can be extremely slow by default.

Community troubleshooting has identified that the Linux Steam client relies on system DNS resolution in a way that performs poorly without a local DNS cache (e.g. dnsmasq). Users have reported significant improvements after:

• Installing a local DNS cache
• Disabling IPv6
• Editing the steam_dev.cfg file (this may not be necessary but users reported improvement doing this)

This setup requirement is not user-friendly and is undocumented.

Expected behavior:
Steam should deliver normal download speeds out-of-the-box without requiring advanced network reconfiguration.

Suggestion:

• Investigate the root cause (e.g. aggressive or repeated DNS lookups).
• Consider including an optional, toggleable DNS caching mechanism in the Steam client itself as a workaround for users on distros without a local resolver.