The Mac mini has 256mb ram which is a little on the small side for OSX with a few apps open. Since the swapfile is on a very slow HD vitual memory performace will be bad.
Compare the same app on a dual G5 with 1GB of memory and a FAST HD and on the mini and you will notice a big difference.
Its just a matter of speed, especialy while multitasking. System speed is affected by quite a lot more then just the processor and video card.