Earlier today I delivered the first benchmarks of Ubuntu's Unity 7 running over XMir to run the current X11 desktop atop the Mir Display Server via this compatibility layer. These benchmarks documented the performance impact of running OpenGL games when having to deal with XMir rather than just a clean X.Org Server running on the hardware. The extra step in the rendering process did result in a measurable performance impact, especially when the performance of the open-source Linux graphics drivers is already lower than their proprietary brethren. The benchmarks to now show illustrate that the 2D rendering performance also takes a hit when running Unity on XMir.
If you didn't read The First Benchmarks Of Unity On XMir: There's A Performance Hit yet, do that first. The testing happened from the same Intel Core i5 laptop with HD 3000 "Sandy Bridge" graphics and using the same Ubuntu 13.10 Saucy development packages when comparing Unity on XMir versus Unity just on an X.Org Server without any unity-system-compositor / Mir.
The testing process is again the same in this article but rather than looking at the OpenGL performance it's looking at the 2D performance using a variety of GTK, Qt, and pure X11 tests. All benchmarking was fully automated using the open-source Phoronix Test Suite software.