The First Benchmarks Of Unity On XMir: There's A Performance Hit
Starting with the id Tech 4 Doom 3 game, running an X.Org Server without XMir was 12% faster for this first person shooter. In other words, there is a performance penalty to running the desktop under XMir rather than when the xorg-server is communicating with the hardware directly.
The most extreme case of the XMir/Mir performance overhead was Nexuiz at 800 x 600 where running it without Canonical's display server was more than twice as fast.
When upping the Nexuiz resolution to 1600 x 900 for the Intel Sandy Bridge laptop, X.Org without XMir was still faster but by a much smaller margin.