I'm not asking why emulators run slow in general. I know there's a performance hit when you're faking being another OS. That's not what I'm asking, but that's all my web searching results.

I mean the game plays fine until it gets to a certain point in a certain level that I remember running slow all those years ago. Like there are 20 enemies on the screen or something and that's the same way the NES would lag 40 years ago.

The emulator is on my PC, which has orders of magnitude more power. Even accounting for emulation's inherent performance hit, it seems like it should be able to handle it.