In looking at the Catalyst driver result data from the Radeon HD 5750 graphics card, GNOME Shell with Mutter is noticeably slower than all of the other desktops / window managers. Similar to some results we had published long ago, the modern Catalyst driver is faster on the GNOME 2.32 desktop when Compiz is running rather than Metacity without compositing. The GNOME2 performance with Compiz was roughly the same as when Unity was running with Compiz and also when the KDE 4.6 desktop was running with KWin and its default effects. Running GNOME Shell with Mutter compositing cut the frame-rate nearly in half.
When the NVIDIA proprietary driver was in use, the OpenGL performance of the full-screen Nexuiz game was effectively the same across GNOME2 + Metacity, GNOME2 + Compiz, Unity, and GNOME Shell + Mutter. The frame-rate was actually better by roughly 12% though when the KDE desktop was running with KWin compositing.
With the open-source R600 driver stack using the Gallium3D version, the best performance was found under GNOME 2 with Metacity while the KDE performance was reduced by about 33% and the GNOME Shell performance was lower by another ~35%. Again, driver bugs made using the Compiz-based desktops fail so those results are not available.
Lastly, using the open-source Nouveau driver resulted in roughly the same performance between GNOME2 with Metacity as in KDE, but the GNOME Shell was the slowest desktop with its frame-rate going down by about 10 FPS, or about 23%.