With most music programs its the number of simultaneous tracks playing that really hammers the processor. The overall length of the track shouldn't have as much impact (Number of effects assigned to tracks can also be a factor). Your mileage may vary . . .
Definitely hook up a velocity sensitive midi keyboard with the software instruments to get garageband to stretch out a bit. In the key of C majore (or A minor) your free to play any of the white keys :-).
I found the application to be very consumer-level oriented and it didn't quite pack enough punch for my needs/wants but they've definitely provided some nice user interface components which I hope get incorporated into some of the higher end multitrack recording apps. Being able to browse software instruments by pictured category and having the pictures in the track view can be a nice little feature for quick track identification. My mind groks pictures faster then text strings :-)