I know this is an old thread, but I made an account just to reply here for posterity. =)
I JUST finished playing Tales of Destiny on my PSP and yes, you do get a lot of freezing by the time you get to Junkland (for the cave quest). By that time I was freezing almost every 10th-20th battle or so, which makes the game VERY difficult to continue. HOWEVER- just turn off the the spells that take up the most fps and you'll do fine! Keep in mind that these spells do not automatically trigger a game freeze, but have the possibility to freeze your game. So while you can cast these spells, there is always a percentile chance that you might freeze your game. So as the game progresses and you get more and more of these spells, the likelihood of your game freezing increases!!
I turned off the following;
Atwight
Maelstorm
Tidal Wave
Chaltier
None (you could turn off Demon’s Lance, but I never had a problem with it)
Clemente
Cyclone
Explode
Black Hole
Tidal Wave
Dymlos
None (I never really used his spells, but 'Explode' would be the one to avoid)
Igtenos
Cyclone
Divine Wind
As soon as I turned these off, I was able to play the rest of the game. It didn't freeze a single time after I turned these off. Also, you have to be really careful when you equip discs because that character may get a freeze triggering spell with the disc without you realizing it immediately, so watch out!! Other than that- Enjoy! ^^