Out of incessant curiosity and borderline obsession - I installed KDE, then I enabled the experimental Wayland support for Proton and tried the game again.

It runs great now. The Wayland support didn't seem to do anything when I was using Gnome but it's a noticeable difference on KDE. As long as I disable vsync which locks the framerate at 30fps for some reason. But overall the game is very playable on Linux now.

Guess I'm switching to KDE.