GarageBand seems to use a ridiculously large amount of CPU time even when idle, which could cause problems on lower-spec systems. This is because it is continuously monitoring the audio input and animating the meter on the selected track.
If you make the Master Volume track visible and click it so it is selected, there is no associated audio meter, so CPU usage drops away to almost nothing! Note I only noticed this once I had upgraded to GarageBand 1.0.1, so it may not apply to the original version.
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Mac OS X Hints
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