the image size could also have some bearing. If the machine is in the process of trying to (for example) resize a 1G file down to 100 mB, it would act as described.
On a completely different line of thought, are there any other applications that are acting like this? (namely FinalCut, Illustrator, or Quark) A friend of mine had a problem with similar symptoms on his ibook G4 800. Applications would open fine, but were not usable and generally resulted in the system acting as if it was hung. (this is in panther). I tried permissions repairs, optimization, and all the normal stuff, but in the end, deleting his user preferences folder solved all the problems. If this is indeed the case, you will be able to tell by creating another account (if you don't already have one) and trying photoshop from that account. If this solves the problem, it is user account based, and I would consider deleting them. (backup as always) Sounds weird, but it worked.