Boss attacks are probably the answer here, which I was trying to avoid because... well I can't remember. You're safe for now though. It will take me a little bit to figure out what I want to do.

However, its not uncommon to get 100+ million in one loop in under 10 minutes, so I don't see this as a terrible thing that needs fixing immediately.

Uploaded unlisted as it sounded like you'd want to fix this probably:
https://youtu.be/mf4NVV3PugQ

Alternatively, the boss itself could start shooting patterns that get increasingly harder.

The way I'd propose fixing it if you don't want this to be possible would be having different spawn caps by enemy type, that increase as the loop gets longer. It'd still be possible to farm end of the loop, but it'd be harder if you still have to actually deal with all the different enemy types.

Yeah theres a solid chance once I post my strat for this, it'll get patched immediately lol. The crux of it lies with the hard cap on how many enemies can spawn at a time, if I intentionally dont kill any red enemies I can max that out, set about killing all non-red enemies until all that is left is a slow blob of dudes in the center. Then, just build to 10x and FEVER, and with the modifiers I had on that's 3.5 million. You then need to survive the next 10 seconds that has a very high spawn rate, but if you get them to clump up again and kill all non red enemies then you're back in multiplier building phase.

Fever does yeah. This seems like a bug. A loop is like 8 minutes at most before the game starts spawning an enemy every frame to kick your ass.

Or, wait, FEVER actually speeds up time.

I'll upload video, but I realized it actually isn't right, that timer is running faster than realtime. Does Extra Fast spawn speed actually make the game time run faster?

What the hell. 36 minutes in loop 1? That doesn't seem right...