I know garageband is slow on the G5's at my school, and whenever you open a small little file it takes like 4 or more seconds to load.Overall garageband is slow and clumsy. Scrolling down the song in the clip view is agonizing, but that's what i would expect since macs have a problem with scrolling. It shouldn't be slow, for a program optimized for macs...
Oh well i could care less, because i used Cakewalk Sonar and pro audio, which allow you to do many more things, including create music from scratch.
Just my 2c...