Several options in GarageBand’s Preferences can help minimize the load on your CPU. Choose GarageBand > Preferences to open the General Preferences window, then click on the Audio/MIDI button. You should see “Optimize for” in the Audio/MIDI pane. The option “Maximum number of simultaneous tracks” is recommended for slower Macs. If your processor is overloading or the playhead turns red, odds are this option was not checked.
On the newest, fastest Macs, you’ll want to have “Minimum delay when playing instruments live” selected. You can also lower the number of tracks and voices per instrument in the Advanced pane of the General Preferences window. The minimum is eight, while the maximum is 255.