I’ve been getting horrendously slow speeds on Linux Steam (~500k/s) and 5-6Mb/s on Windows, and only now found out why. There’s a ticket on GitHub for this:

https://github.com/ValveSoftware/steam-for-linux/issues/3401

In short, the client is very aggressive on its DNS requests, which normally causes it to be throttled by servers, leading to really slow downloads. However, using dnsmasq allows the requests to be cached locally and offload the requests.

Even though the instructions are for Arch, they worked for me:

1. Install dnsmasq
2. Modify /etc/dnsmasq.config and add the line listen-address=127.0.0.1
3. Restart the dnsmasq service (systemctl restart dnsmasq.service) or reboot your machine

Enjoy the speed