I know about the buttons, and the reason for this is that when you rewinding or stopping time, CharacterController needs to be disabled. Buttons use OnTriggerEnter and OnTriggerExit to test whether they can be pressed. For some reason OnTriggerEnter is not executed when enabling collisions. The controls were done this way on purpose.
I don't know why level 4 is so laggy. I don't like this level either. I had no idea for it, so I did whatever I could think of. I will probably change it a bit in one of the updates.