For what it's worth, using the solus steam runtime (SLI) I see noticable improvements over steam-native.
When I installed Solus around 4 months ago, steam was noticably faster on the same hardware than it was when installed on Ubuntu.
There are probably ways to get the same experience on Ubuntu, but if I were a non-technical user I think I'd only really care about which gave me the best experience out of the box.