I really liked it. The art style is great and the music is catchy.
I think some basic feedback would go a long way. Such as when a hit of yours connects, give some particles. Similarly, some indication of you being hit. I know there are sound effects, but sometimes the overlap of so many grunts and the music made it hard to hear. Similarly, when a fireball is ready some like shine that you can fire it.
Performance wise, I have an FX-8350, 16GB RAM and a GTX 1070. I had a ton of dips into single digits, then it was running at 150fps. My guess is an issue in instantiating all of the enemies. Especially at the start, a huge FPS dip when you're likely making all of those new enemies. There's an article about deactivating and then recycling assets if you need them versus always instance and queue_free, but that's just my gut feeling.
It's a great POC and the lighting/design really stood out to me. I'm sure as you add more features and mechanics, enemies, locations, etc. it could be a fun dungeon crawler!