Very good impression overal. Gunplay and controls are already pretty nice, with some polish and refinement they should be perfect. Visuals are excellent and design is consistent. It's easy to tell one enemy from another, altough the tanky grey demon threw me off a bit the first time I encouneterd him. I took him for the regular melee guy.
There's way too much ammo on normal mode - more limited resources might result in a more tactical approach and more interesting gameplay. But it depends what you're aiming for.
The level design rewards exploration but it's perhaps a little too complex and confusing. I first stumbled right at the beginning because I didn't notice the ladder next to the gate. There were multiple times when I had problems noticing buttons and corridors. On second level I got so lost that I gave up. It's good that game lets you figure out stuff but I think finding a way through the level should be easier than finding secrets.
I like the katana alt attack - the cooldown would be a good idea to use it strategically, if there wasn't so much ammo lying around at player's disposal. Also - since katana has an alt attack, are you planning on adding alt fires to other weapons?
I'm not entirely sold on the bullet time. It sounds cool but I don't actually feel the effect during combat. It feels very random when it happens, especially upon killing last enemy in a fight. It might make more sense to have it as a colldown ability under a hotkey. And maybe if it for example turned hitscans into slow moving projectiles for the duration of the effect. Then it would make sense to put high damage hitscan enemies into the game.
I never once used dash in combat situation. I only used it for backtracking and obvious platforming segments. It could be a really good tool for quickly reaching high priority targets, especially combined with bullet time. If there were hitscan enemies dashing could be used to make them miss.
Enemy infighting is always a nice feature - something to consider:)
Spiders - adding small, fast enemies with tiny hitboxes to the game is risky - people usually hate them with passion. So keep that in mind and make sure to give the player some good AOE weapon that would make them lot less annoying:) And since you already have spiders, add some giant buffed versions of them for the sake of variety!
There might be some optimisation issues as well - Game runs below 30fps on my PC - which is far from high end but in the same time game does not seem to have dynamic lights (aside from muzzle flashes), ssr or any other expensive effects that would explain this.