Does that mean this is resolved?
If not, you might also take a look at your mem split setting. You can find it in /boot/config.txt. I had mine set to 256 (on a 512mb pi B) and was getting a lot of slowdown. I reduced the memsplit to 128 and the same games are completely playable.
My issues were with FBA, but the same might apply to NES and SNES? For what it’s worth, NES and SNES games seems to run fine for me at 128, with no overclock. I didn’t try them at 256.