It was the saved game, thank you so much.

My installation is not legitimate, but I haven't modified anything in the installation directly as far as I remember. I recently did something with Visual C++, something about installing a different version between the last time I played the game and when I played recently, and my brother also has an illegitimate version, he recently got the launcher and donut mod and he seems to have no problem playing with it, similar computers and all.

Was having the same problem as OP, frame limiter fixed it for me with limit at 60 frames and limited while on loading screens. Does this fix it for you?