Phoronix: Mir Got An Important Rendering Performance Fix
The latest revision to Mir features an important performance fix...
Phoronix: Mir Gets Fixed From Serious Performance Issues
While Mir is big onto integrated testing and was designed from the ground-up this way, performance regressions slipping into the code-base doesn't appear to be incredibly uncommon.
Phoronix: Nouveau 2D Rendering Is Much Slower Than Intel SNA
Besides Nouveau's 3D performance being very slow until re-clocking is figured out, the 2D performance of this open-source NVIDIA Linux graphics driver is also very slow...