I assume you've already watched darbian's tutorial and are asking about the 3DS specifically? the only tutorial you really need is

By speed do you mean that the SOUND is fast? because if so - I had that problem and the issue was that it was a PAL rom not NTSC

Congratulations Darbian! Was that a perfect run using current non-TAS strats or is there ANY room to save frames now?

Switched to NTSC rom and... it's all normal - I can't delete the thread - but, all of my concerns apparently only apply to the PAL rom. Case closed.

Found a YouTube video illustrating the difference (not mine)

THAT would definitely make sense - hmm, let me try a different rom

There are lots of other ways FCEUX (at least the two versions I've played on my two computers) differs from NES hardware...

¤ Lakatu stays on the left side of the screen during 8-1 (if you're perfect pace), whereas on NES he doesn't ¤ While "running" in 1-2, you won't land on lip of the second pipe, unless you slow down, on emulator - on NES you usually do ¤ on 8-2, the intro strategy doesn't work with lakatu's dropped baddie - you always run into him (when jumping first on the second block) - but on NES it's fine. ¤ I still hold that forward jumps move your xpos more in FCEUX than it does in NES, but I haven't conclusively shown that - although I rarely don't get the WW on emu, and am still very bad at it on NES... that and look at the first video of 1-1, I tried that on NES (but didn't record) and didn't move as far forward.

It appears that there are some pretty clear differences between emu and NES, at least from what I've seen... and I'm sure that there are more, I haven't really "figured out" the speedrun past 8-2, so I don't know of any obvious discrepancies beyond that point. I'd do a "TAS" style input comparison, which would be a much better test, but I have no idea how to do that on the actual NES hardware.

Did it on AVS with the original cartrige, not the powerpak. The forward jump advancement is WAY bigger on emulator than on NES

Here's the video from the original post getting the wrong warp in 4-2 without any backwards jumps

Skip to 0:38 for the run without backwards jumps

Here's a video of FCEUX I made from 1-1 doing forward jumps, got to maximum x value a little before the "stairs" - tried it on NES (AVS) and couldn't replicate.