Newer Portage versions are able to use the installed confcache tree.
Just emerge confcache and add "confcache" to the FEATURE line in make.conf.
This makes portage use the cache and most configure runs will become faster.
There are a few problems when using confcache on some ebuilds. Check out http://gentoo.linuxhowtos.org/portage/dev-util/confcache for all known issues.
Happy (faster) emerging.