Abnormally slow speeds downloading from Steam

Might be a silly suggestion but have you tried going into Settings and checking whether there's a bandwidth download limit set on your account?

It's found under the Downloads tab. Once clicked, there should be a little dropdown menu.

I had the same problem and the dnsmasq solution did not work for me, neither did changing download server or set a download limit. I hope this will help others in the future:

The problem for me was that my steam folder was on a mounted ntfs partition with sync enabled (check /etc/fstab). That throttles write-speed significantly. Change sync to async and I have normal writing speeds which means Steam can download at full speed, too.

On Linux Mint Cinnamon 19 (and possibly most of Ubuntu based OS) :

In a terminal :

sudo nano /etc/sysctl.conf

In nano editing add this lines at the bottom of the file :

net.ipv6.conf.all.disable_ipv6 = 1

net.ipv6.conf.default.disable_ipv6 = 1

net.ipv6.conf.lo.disable_ipv6 = 1

Save and close

In the terminal :

sudo sysctl -p

Sources : https://forums.linuxmint.com/viewtopic.php?t=210093