Most of my friends (and myself) didn't actually have lag on the jam version but obviously it stood out to people that did. I'm pretty confident it runs much better no matter what now.
For the story, we had actually already profiled before releasing, but... the bottleneck was Unity's SpriteSkin OnEnable, called once per enemy spawn, rather than one of our scripts, as our animation is done with 2D bones.
Some people solve that by not deactivating the object when returning it to the pool and just deactivating every component but the SpriteSkin.
I opted for the non-insane way of baking the sprites' animations instead.