Garageband should be able to handle much more than this in terms of length. Multiple tracks are going to stress out your Mac far more than long songs will. However, there are some things you can do to make it easier on your Mac:
Software instruments
These are tracks that show up as MIDI bar charts and not audio waveforms. Think of MIDI as analogous to source code; The Mac has to essentially compile the MIDI into audio data before playing it, and it has to do this in real time.
This becomes more of a problem when you have several such tracks.
What you can do:
Click on the lock icon before playing the track, and the Mac will pre-render the audio before you play it, essentially "compiling" it ahead of time. You'll have to wait a minute while the Mac does this. If you want to make any changes to these tracks, just unlock them.
Effects
The Mac has to render the effects in real-time when you play audio tracks. If you have a lot of tracks with multiple effects (compression, reverb, chorus, delay) then you're slowing things down quite a bit.
What you can do:
Similar to software instruments tracks, you can lock these before playing them. (Can anyone confirm if this locks in the effects as well?) But do you really need all those effects?