Google a bit for adjusting GC settings in idea - stuff like http://blog.jetbrains.com/idea/2006/04/configuring-intellij-... (old, but check the comments). I'm not using it anymore, so can't find what I ended up with, but adjusting the startup options made a huge difference 2 years ago.
I think I also went for one of the experimental/less stable concurrent collectors - didn't find any issues in practice.