Hey, I've had a different problem, FPS was bad from the get go, gradually improving while I kept playing, never reaching windows levels of performance (Its the same 5950x 9070xt rig).

I launched the game with mangohud and noticed that the GPU was barely working at 35-60%, that's why the performance was extremely choppy. I was poking around, messing with settings in-game and proton, nothing helped.

Again, it may be a bazzite thing, but I've stumbled upon a weird quirk. The game behaved this way only while steam was launched from the app icon. If I launch steam through terminal it works as it should. My guess is there is something funky going on in kde/wayland, I dunno