Most snes games need some tinkering to get running at full or acceptable speeds, but its usually possible.
PSP accel. + approx. soft. and PSP accel. + accur. soft. usually get it done, also vsynv off, frameskip auto 1 and limit frames to 60fps work for most games, but sadly not for the cool ones that had extra chips (kirby, yoshis island...) or those that heavily use mode 7 (mario kart, some FF)
FF6 on gba (uo gpsp kai) works but only using frameskip. it'll run at 45 or so fps, but it will look mostly normal, its not that big a deal in rpgs after all. some slowdowns in places with many npcs (mogs protecting terra for example), some attack animations (mass heal i think, though i believe that caused some slowdown even on original hardware) and I only played it until you reach sabins mountain, dont know if having a full party affects frames in battle.
having said that, you might want to look into the ps1 port of the game. it should run at full speed, but with annoying loading times when entering towns/battles/using skills. i never tried it and i'm not sure if loading times were only due to the game being on a disk, or if they were hardcoded into the game.
in general, you have to understand that the psp really isnt that powerful and final fantasy 6 is among the games that took everything the gba had to offer back in the day. it running as well as it does is kind of surprising tbh.