Yeah, I was developing it on a reasonably powerful PC, so I have no idea how poorly it runs on mid-range or low-end hardware. My PC can handle around 400-500 enemies before I get any kind of frame dropping, so in the game I capped the number of enemies at 200. I was hoping that would be good enough for most people.
I'd say Magic Blast and Smite got the most use while I was testing. Magic Bullet is really good if you get it to level 5, especially if you combine it with the projectile upgrade.