I absolutely loved this game, proper old school feeling with the sounds and the gameplay!
First round I was afraid that it would be a bit slow/boring, but then there were new enemies introduced and small bosses and then in the end the ultimate boss! Absolutely great work!
Few suggestions: Maybe the timepower only affects enemies, to give you some advantage over them for a while, at least that you could move normally.
Other suggestion would be to nerf the gun a bit, maybe not the damage itself but put like a overheating for it, since I played it through with just holding the left mouse button, if there was a gauge that filled if you hold the mouse button on for too long you would have to wait for a while for it to cooldown to be able to shoot again. Obviously this would affect the whole gameplay and probably need to do some slight adjustements to everything.
But overall great game!