Sounds like a lot of black magic, but there’s a thread thats linked from the GWN..
http://thread.gmane.org/gmane.linux.gentoo.devel/23833
Some of the more promising ones look like:

• Mount /var/tmp/portage on a ext2 filesystem with async and noatime
• Don’t use ccache if you don’t need it

Perhaps I can add a third:

• Use debian 😉

But in all seriousness, gnome 2.8 on debian seems to run a lot better than my gnome2 on gentoo at home. And its more up-to-date now.. is this because of Ubuntu and Canonical?