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.
The NES can only render 8 sprites per scan line. Games knew about this, and some, like Zelda, would double render the frame if it detected more than 8. This has two effects: Some sprites would flicker, as they’re only being drawn on one of the two passes, as the frame takes twice as long to draw, resulting in a slowdown. This behavior is part of the game, and would need to be patched out if it were running on an emulator that does away with this restriction. (However, because the emulator is no longer 100% faithful to the original hardware, some people might choose not to use it.)