The Xubuntu developers put together a gdm theme that will load xfwm4 instead of Metacity. To take advantage of this little boost in preformance, install the following package:
sudo apt-get install xubuntu-gdm-theme
Unfortunately for us running a lean version of Ubuntu or Xubuntu, the theme needs a little help beyond just installing the theme:
sudo rm -f /usr/share/gdm/autostart/LoginWindow/metacity.desktop
When GDM gets an update, you may have to remove that file again. I haven't played with it enough to figure out how to disable it permanently, if anyone has any suggestions, I would appreciate it! Just post them in the comments section.