Unity3D in Ubuntu 12.04 has a regression that leads to slower frame rates in many 3D Linux games. This happens under Unity3D only and other sessions like Unity2D and Classic Gnome are unaffected.
The good news is that a fix for it has been committed but it may take days to officially land Ubuntu 12.04 updates.
If you want to get this fix now, update Unity from PPA by running commands below:
sudo add-apt-repository ppa:benkai/precise-unity-stable-branch
sudo apt-get update
sudo apt-get upgrade
After the update, there will be around 20-30% improvement in frame rates of 3D Linux games.