The problem is that the default might depend on your installation. Did you install RetroPie using the setup script yourself or did you use the SD image? If you did use the SD image have you updated everyting using “binary install” e.g.?
There are currently over 30 emulators that RetroPie runs. I’m guessing you are using a Super Nintendo emulator of which there are 5. of those 5 there will be varying performance depending on the game/emulator. If you are using a Raspberry Pi 2- the preference would be lr-snes9x-next.
Then you also have to take into account your rom- it is possible the source you got it from was a bad dump.