I feel like that is a steam side thing.
Searching the interweb shows signs it might be a dns thing, but it seems inconstant so I think that is just a false positive if it works.
I would not be surprised if the speeds are limited by valve because you are using linux.
Some suggest switching to flatpak version of steam. I question that.
This user f.ex followed the advice to switch to dnsmasq but without success. Then it just suddenly started to work after a while. Pretty sure it had nothing to do with dnsmasq.