This game is actually really fun. The lag is annoying, but it's improved by reducing the program priority back to normal. (That's right, increasing program priority makes lag worse, because it sucks cycles from the keyboard and makes the game slow to respond to my input). The level selector is odd; a more traditional interface might have been better. The graphics look very cheap at first sight, even though they're not.
Yeah, some computers seem to fair worse than others when it comes to the level select in terms of performance. Unfortunately, this is because I had to do the collision / solidity checking manually in this game due to the limitation of Game Maker's built-in "solid" system, which doesn't work when you're alternating between two different versions of the level
I've optimized everything to the nth-degree after getting feedback about this, so it's practically running as fast it will ever run. There's not much else I can do about it I'm afraid