Ah, wonderful! My domain specific problem for my version of this gameplay, was the severe blob growing in the center of the screen. It didn't help that my game isn't optimized so well! Your game is butter smooth :)
In that regard, there were two main decisions I made that helped reduce lag:
- I tried to design the enemies such that they were less plentiful but a bit bulkier/more aggressive so that there was less "crowd crush" to resolve
- To account for cases where the player isn't clearing enemies fast enough, or even intentionally avoids damaging enemies to amass a bigger crowd, I made it so that any enemy who lives for longer than 60 seconds will start to run away from the player instead - and despawn once they're out of the play area. This did have a side effect of having some enemies run away and then fire at you from way outside of the camera region, which I forgot to fix haha
The TL;DR is that even without a super efficient collision method, some effective game design can go a long way to help circumvent the issue