Well…Looks bad, looks like it’s specific game + proton problem after all.
Do you have any other game to test and see if it’s an anomaly?
Let’s try the last thing - ReShade, it may help although one might think it’s counter intuitive.
Follow this steps 1-5 (just don’t use shaders, all you need is ReShade itself because it might help you smooth out performance crap of that game)
When done just press HOME button in game, if ReShade panel opens and there are no errors - you’re fine to test if it helps.