I've had iTunes consuming a majority of the CPU. There must be a thread in there that has gone haywire. This is particular a problem when burning. I burn a lot of coasters because of this run-away thread that competes for CPU with the burning thread and I end up with a lot of coasters instead of playable CDs.
Dropping the priority on iTunes doesn't really fix the problem with iTunes. It just sets the priority lower so that they runaway thread doesn't get as much CPU and other processes that need the CPU will get it instead.
I wish Apple would fix this bug, because it still plagues even the latest version of iTunes that I'm running in Aug '09.