The branches of trees/treants must just pretty complex to render or something, it is likely not be fully optimized. In 1.0, if you had any of the event christmas trees on screen at all most people's computers would start to chug and heat up, and turning the camera to get a view inside them would make the game stall like mad. The same issue happened when they released the moogle fight where the entrance area had a lot of treants. In both cases, the issues got fixed at some point so it shouldn't be an issue in the long run.
I remember turning off shadows helped a lot in those instances, if the PS3 has that option you can try that?
