In my tests with the compositor, I’ve noticed my 4-core OSX machine go from 400% rendering in the activity monitor during the normal render, down to 100% during the compositing phase?
Is the compositor multithreaded? If so, how can I enable it?
If it’s not multithreaded, is that on the list for 2.5? 
Sorry if this has been covered in previous posts. I did a search and couldn’t really find a definitive answer.