The Unity desktop in Ubuntu 13.04 supports a different rendering mode that will increase performance on lower specced machines. To enable this, enter the command below to create an empty file in the /etc/X11/Xsession.d directory.
sudo touch /etc/X11/Xsession.d/99force-llvm
Then type sudo vim.tiny /etc/X11/Xsession.d/99force-llvm and enter this text.
Then press ESC and type :wq and hit enter to save and quit VIM.
This will force the Unity desktop to use the low graphics mode when rendering the desktop and this results in higher performance. Just log out of the Unity desktop and then log back in to see the changes.