I’ve heard people say that the linux/proton performance of CS2 is shockingly bad but I haven’t really investigated yet. BG3 is more shocking but that is also a CRPG which is a genre known for engine jank.
Aside from what others have mentioned: Getting more data on what is falling over helps a lot. Watch your favorite computer/tech youtuber and look at the kind of info they have in their benchmarks (frame time, etc). Personally? I am too lazy to look at systematic ways to collect this on linux but I do like to use mangohud. Set it up to show me some useful metrics and toggle it on for games that are performing unsatisfactorily. Generally lets me narrow down if it is CPU, Memory, or GPU limited and even what might be misbehaving.