Linux+Proton: Input lag starting consistently in my second game

[Hamster]

When I play on Linux over Proton (no mods installed), I always have one smooth game, and consistently in my second game, usually a few minutes into it, my game starts madly jittering making it very unpleasant to play.

The jitter doesn't seem to be caused by a drop in framerate or any such performance problem, but by my cursor starting to move like it's on a quarter-second sine-wave, leading to my rotational movement feeling very jittery. I verify this by opening the escape menu and moving the cursor over it, and it'll be phase-jumping over the screen. When I'm not rotating, in-game animations are smooth and everything looks fine.

Does anyone know what might be causing this?

[Hamster]

Actually, maybe it's not just my cursor. It seems my character animation is janky when normally walking and switching weapons. It's just that everything else in the game world animates smoothly.