welll it's definitely a kernel level issue considering it happened the same on amdgpu and nvidia proprietary drivers, i didn't test with nouveau but if it happened on amdgpu odds are it'd happen on nouveau as well. But using zink (it's like dxvk but for opengl instead of directx) somehow solved the issue.
It'd be an interesting bug to report, but honestly what am i gonna say? "Hey Linus, this renpy hentai game lags like crazy on linux but not on windows, plz fix"?
I suppose it could also be an issue with renpy itself, but i doubt it strongly because then running the game through wine should have fixed it, but it didn't.