Actually, you can ignore this. It seems the problem is either my OS was outdated or my HDD was already breaking down. There are just some areas in the demo that seems to lower the FPS but it's still playable, IMO. Would there be a sprint button if ever you wanted to implement this on the game and demo?
Edit:
NVM about the sprint. You already answered it. In any case, I suggest an option for windows mode and probably some resolution options if you haven't done it yet.