With the CPU-bound OpenArena game, running it with Nouveau under GNOME Shell and Xfce were the best. The slowest was using the Qt-based Unity 2D while the results from the other desktops were mixed. The binary NVIDIA driver was the same story in terms of relative desktop performance, albeit multiple times faster than the open-source reverse-engineered driver alternative. The GNOME Shell and Xfce desktops also did very well on the open-source Radeon driver. With the Catalyst driver, the performance was the same across all six desktops with very little spread.
World of Padman, a game based upon the ioquake3 engine like OpenArena, was basically the same story.