Avoid the PSP port of FFT. The game is programmed to intentionally slow down to 15 FPS during most animations making the game unplayable.
Truly spoken like someone who has never played the US port of the game.
Spell effects are shown slower on the PSP version - probably 25-30% slower - but this is made up by the effects taking less time to load than the PS1 version of the game. In addition, the stages and menus also take less time to load, so the whole thing actually balances out in the long run.
It takes very little time to get used to, and overall you're losing no time playing FFT:PSP over FFT:PS1.
In addition, the revised translation, extra stages, extra characters, new plot details, and other features really make the game shine over the original.