I – among many others – had big problems with download speed under Steam for Linux. The games sometimes only dripped through the line byte by byte, while the line itself showed all fine when downloading stuff elsewhere. There seem to be two problems with fixes which work for many people. I’ll show you both and recommend one of them.
But one thing beforehand: Sometimes, Steam’s servers are just overloaded. GTA V is new on PC? Call of Duty for free this weekend? Chances are you won’t get fast downloads whatever you do. But sometimes, neighbor region’s servers might be doing better. Then you can choose another server region in the Steam settings. This of course works for Mac and Windows as well.
The Steam client seems to query the Domain Name Server very often. This can slow down downloads even despite a good DNS connection. The workaround suggested is installing a local DNS cache. As the IPs queried are the same every time, they easily can be provided by a cache.
Installing the DNS cache under Debian:
aptitude install dnsmasq resolvconf
Installing the DNS cache under Ubuntu:
sudo aptitude install dnsmasq
This hint has been taken from here.
File sync flooding
The other problem of the Steam for Linux client are too many calls to file synchronization functions. These force the system to write to disc what it has got at the moment instead of gathering more data first. That’s especially problematic with a feature called Write Barrier. Many people got rid of the download problem by disabling the feature in their /etc/fstab file. But doing so disables a safety measure of the file system. (See tux9656’s comment below for a possible alternative.)
This is a sample entry:
UUID=xyz /mnt/SSD_2 ext4 defaults,barrier=0 0 2
Caution: Editing the fstab file can render your system unbootable!
This hint has been taken from a hotline article of the German computer magazine c’t.
I tried both workarounds. The sync flooding change did not help, the DNS flooding change did. I undid the file system change. My download speed is still not always as fast as the line, but that might very well be due to Steam servers – and it never got down to some bytes per second again.
I would suggest to try the DNS change first. Not because it was the one that helped me, but because it is a useful change to your system anyway. If that does not help, additionaly try the other one. Please mind that it results in a system a bit less safe.
Of course, you’re at your own risk with both hints. You have been warned.