Since you have a 3DS XL, I guess BlargSNES or Retroarch's CatSFC. BlargSNES runs at full speed for what games it does support (or nearly full speed), but it's compatibility is a bit weak last I checked. CatSFC has a minor annoying issue where the instruments are higher pitched, but it might be hard to notice and is less demanding on the non-New systems.
Otherwise, if you're using a New 3DS XL, try CatSFC Plus. It's the same thing as CatSFC but has the sound issue fixed. PocketSNES is also on the same level of quality, but I have no clue if there are any differences between the two.