I've played Mario in emulators where I could play just fine, and others where I kept falling down holes and such on friggin' level 1, which at times I could probably have beaten literally blindfolded, and had a hard time progressing at all. The latter might not (or, if extremely bad, might) be laggy enough for me to be able to tell you by looking, but plainly the feel is off enough to be a problem.
I find a good test is Punch Out!!! If it's much trouble at all for me to reach at least Great Tiger, the latency is really bad (even if I couldn't tell you just by looking). If I can get to Great Tiger without much trouble but struggle to do much damage to him before getting taken out, the latency's still unacceptably high for some games, but not totally awful.
Another good one's High Speed. If I can't land the final multi ball shot at least a decent percentage of the time (the game pauses the ball a couple times while police chatter plays, when you're set up for a multi ball, and after the last pause you can land the shot to initiate multi ball immediately and skip all the flashing-lights-and-sirens crap if you're very precise with your timing, it's like very-small number of milliseconds after the ball resumes its motion) then the latency is high enough to affect gameplay.
If I can land that shot at least 60-70% of the time, and if I can reach Bald Bull in Punch Out!!!, then probably any trouble I have in other games is my own damn fault :-)
I suppose as I age further these tests will become kinda useless for me, because my reflexes will be too shot for me to ever do well at these no matter how many hours of practice I've had over the decades :-(
Anyway, even in the best case you're always going to have worse display and input latency on a digital screen with a digital video pipeline and USB controllers than an early console hooked up over composite or component to a CRT. I've found it's low enough (even on mediocre TVs, provided they have a "game mode", and those are a ton worse than most PC monitors) for me not to mind much if the emulation itself is extremely snappy and is adding practically no extra latency, and there are no severe problems with the software side of the video & input pipelines, but otherwise... it can make some already-tough games unplayably hard in a hurry.
I do wonder about the experience of people who try these games for the first time in an emulator. They'll come to the game with no built-in way to tell if they keep slipping off ledges because the latency's like six frames instead of the ~ one it was originally, or because they just suck at it.
I find a good test is Punch Out!!! If it's much trouble at all for me to reach at least Great Tiger, the latency is really bad (even if I couldn't tell you just by looking). If I can get to Great Tiger without much trouble but struggle to do much damage to him before getting taken out, the latency's still unacceptably high for some games, but not totally awful.
Another good one's High Speed. If I can't land the final multi ball shot at least a decent percentage of the time (the game pauses the ball a couple times while police chatter plays, when you're set up for a multi ball, and after the last pause you can land the shot to initiate multi ball immediately and skip all the flashing-lights-and-sirens crap if you're very precise with your timing, it's like very-small number of milliseconds after the ball resumes its motion) then the latency is high enough to affect gameplay.
If I can land that shot at least 60-70% of the time, and if I can reach Bald Bull in Punch Out!!!, then probably any trouble I have in other games is my own damn fault :-)
I suppose as I age further these tests will become kinda useless for me, because my reflexes will be too shot for me to ever do well at these no matter how many hours of practice I've had over the decades :-(
Anyway, even in the best case you're always going to have worse display and input latency on a digital screen with a digital video pipeline and USB controllers than an early console hooked up over composite or component to a CRT. I've found it's low enough (even on mediocre TVs, provided they have a "game mode", and those are a ton worse than most PC monitors) for me not to mind much if the emulation itself is extremely snappy and is adding practically no extra latency, and there are no severe problems with the software side of the video & input pipelines, but otherwise... it can make some already-tough games unplayably hard in a hurry.
I do wonder about the experience of people who try these games for the first time in an emulator. They'll come to the game with no built-in way to tell if they keep slipping off ledges because the latency's like six frames instead of the ~ one it was originally, or because they just suck at it.