This game is really fun! I like how unique your powerups are, especially the one that gives you allies and the ones that come with drawbacks as well. You've got great juice and game feel, awesome sounds and visual effects, as well as screen shake and a really reactive UI. I particularly like how the bullets drop when you use your ammo. Sounds being different in slowed time is a wonderful detail, as well as the addition of a little music. You've got good enemy variety, and the red enemies that ram you really force you to use the time slow mechanic.
Two pieces of feedback from me: I think the normal gameplay should have some kind of music track. The majority of the damage I took as well were also at the start of a round where the game would freeze for a few seconds, and suddenly I'm surrounded by enemies, usually already been hit by the red rammer. I can only make assumptions since I didn't make the game, but I wonder if spawning all the enemies at once at the start of a wave is causing the game to hitch. If you haven't already, I'd look into using an object pooling system for the enemies. Rather than spawning them and destroying enemies, the game would load them all and hide them up front. When you need an enemy, the game unhides them, and when they are destroyed the game hides them and puts them back into the pool to reuse. When you need objects to be spawned and destroyed often like in this game that can really help performance.
Awesome job!