Thank you, I'm glad you enjoyed it :)
The game crashing when you beat the third level is known, since there's no fourth level. The game shouldn't crash when you die though, it should restart the level. Unless I broke something just before submitting.
The framerate plumetting on the third level is due to the collision system and the sheer number of objects. I spent the last couple of hours trying to optimize it, but ultimately gave up and submitted as-is. I'm certain it could be improved. I tried ignoring collisions for objects off-screen (since it wouldn't impact the player experience much), but once you get large enough, all the objects are on-screen anyway. I also disabled collisions between any two non-player objects if one of them is less than 1/100th of the player's size.
The ECS architecture I'm using should make it pretty easy to multi-thread the collision system, but I just didn't get around to it.