Okay, I noticed some issues with your game. I'll try my best to articulate them.
1. The player speed is too fast!!
The map design is much too claustrophobic for moving at these speeds, making it extremely easy to miss all of the nice details in the map and get caught up on level geometry. Mostly frustrating.
2. The dash seems pointless and overpowered
I feel as if the game only suffers from including this feature. The map layout has optional gaps that aren't traversable without using it. Gaps that could easily just be shortened or covered with a bridge. Taking the dash out of the game wouldn't change much in terms of fun factor because it's only situational at best in its current state. Additionally, the enemies are incredibly easy to dodge if you dash out of the way, (Even on the hardest difficulty) so fights turn into a war of attrition very quickly with mostly backpedaling.
3. The bullet-time needs work
It's a cool concept, but in practice, it's incredibly hard to engage because the enemy heads are so tiny, With that, There's not much benefit to using it, since the guns are already so effective. Instead of giving it to the player right at the start of the game, perhaps it should be in the last quarter. I don't know. It just seems out of place.
4. Movement is very stiff
This one is a bit difficult to describe but, when coming down from a jump, you have little to no momentum once hitting the ground. It's incredibly jarring since you can't air-strafe, and build up speed like in Quake or source games. This one would need more testing if you decide to lower the player speed as well. If there was a gradual increase which slowly brought the player to max speed, then that would feel so much nicer.
5. Repetitive enemies
There are way too few textures for enemies. I suggest making more variations. Additionally, the enemy sounds are all the same per type. Should have more variation to prevent it getting boring.