I've been using Eclipse Galileo for some time now and have quite a few favourite (and necessary!) plugins installed. These included m2eclipse, subeclipse, findbugs, checkstyle and many more.
I had noticed though that things started to slow down. Even clicking between tabs was slow (started out ok after launching and then got worse over time).
This smelled like a memory performance issue so I re-configured Eclipse to have a maximum memory size of 2GB and all now appears well. 2GB may have been an over-reaction - your mileage might vary, but for me it seems to have done the trick.