Hmm, I have no experience with gta on linux. Have you found anyone else with this issue?
#please help gta 5 has terrible fps
unreal shoal
This only happens after alt-tab?
What runner/launcher are you using?
What desktop environment? What proton version?
It looks like almost everyone is having issues with gta V on linux https://www.protondb.com/app/271590
It's probably just the way it is for now
tbh gta doesn't even run that good on windows either