Past few months I had terrible download speed with Steam. Games took days to download, and I rarely played games because of that (not true, GOG for the rescue!)
Open a terminal emulator window, and use this two commands:
$ sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1 $ sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
It's a one time deal though, after restart you need to disable it again. I didn't make it permanent, just when I download Steam games, the rest is fine for me.