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Globe of Death

That could be the problem, I didn't even think about that. I know when I first created the level, enemies would spawn falling through the map; so I had to duplicate the bottom of the ball and move it down a few inches as a sort of safety net. So the lag could very well be caused by the enemies constantly clipping through the level geometry.

Maybe it's not the number and type of enemies, but the level itself? Maybe they are interacting with this “Ball” in the wrong way somehow?

Yeah, its probably due to the fact I put some sort of outrageous number on the wheel enemies spawning lol. If the problem persists I can go back and lower the enemy count.

The map is interesting, but, unfortunately, unplayable on my system.

In endless mode literally on the third minute FPS dropped to 15-20 frames and lower and didn't rise to normal values (In single mode, the problems started at the 8th minute).
Lowering the resolution and disabling blood and gore did not help to improve performance.

In fact, the game is well optimized, and problems only arise in infinite mode, when there are VERY many enemies. But this is the first time I've encountered such a major performance problem.