If you are using the Unity desktop with the Compiz compositing window manager, soon as you upgrade your system with all of the stable release updates, you will now have unredirected fullscreen windows by default.
With the first post-Christmas SRU to Ubuntu 12.10, pushed was Compiz 0.9.8.6-0ubuntu1. The first change as part of this upstream release was "Enable Unredirect Fullscreen Windows by default. It seems to be stable enough with all the major graphics drivers now, as at Ubuntu 12.10 (with updates)."
This work is part of this Launchpad bug report. Within that bug report are user results of the new Compiz when using our own Phoronix Test Suite and OpenBenchmarking.org where they are seeing 50%+ performance gains.
Having fullscreen windows draw directly to the screen rather than an off-screen buffer is able to yield some measurable performance improvements for many OpenGL games. Up until now for Compiz it's been an off-by-default option hidden behind the Compiz settings manager and there's also a similar option for KDE's KWin window manager.
There's also a slew of other performance improvements in Compiz for Ubuntu 13.04, but that work isn't related to today's fullscreen window change.