So, for some reason, I was playing Undertale yesterday, and it ran fine, at 30 fps. But when I tried to play it today, the game goes below 20fps. I don't have any controllers connected to my PC, or have any other program running on the background. Any suggestions?

AHAHAHAH try lowering the resolution.

I try playing it windowed, and nothing. Still below 20.

If yesterday was your first time, my guess is that some config file it created has something that's causing problems after loading the game again. You could try deleting all config files to see if that helps, but be careful to not delete your save. Where those files are and which are safe to delete, I don't really know because... Unfortunately I don't have the game installed so I can't talk more specifically, but this sounds like a plausible explanation to me.

mmm weird

I'm not an expert but since I doubt you are playing this game on a microwave screen I guess is having some compatibility problem, which may be hard to solve actually, but is strange that the game ran well yesterday and not today...

must be a config or something but still funny as hell Undertale to drop below 30 fps.

Okay, so I restarted my PC, and now works fine. Mysteries of thechnology, I guess...

You made this thread before making the most basic step when something doesn't work? XD

Many problems I had with my PC have been solved doing that, so it doesn't surprise me.