After I upgraded to After Effects CS5 recently, I found that my rendering was a lot more sluggish than it used to be. Thankfully, I found this FAQ item on Adobe’s forums, and following the suggestions there — especially those about recommended memory-per-core settings — really got me back on track.
For the record, I’m working on a MacBook Pro with a 2.66 GHz Core i7 processor (that means 4 virtual cores on top of 2 real cores, due to hyperthreading) and 8 GB of RAM. The best balance for me ended up being assigning 1.5 GB of RAM to other apps, and 1.5 GB of RAM per render core, with 3 cores available for multi-processing for renders. Lower than that, and I wasn’t getting very good performance; higher than that, and it was even worse, because AE started paging RAM out to disk and that was killing performance.