~12% worse CPU performance compared to non-glvnd setup

After switching to glvnd, I noticed lower performance due to greater CPU overhead.

Benchmark	Legacy	GLVND	Summary
glxgears	~28000 FPS	~24000 FPS	14% perf drop
viewperf13 SW2012_rallyCar_AO	201 FPS	179 FPS	11% perf drop

glvnd spends a lot of time in libGL.so and some in libGLdispatch.so.

The "off" results are with LD_LIBRARY_PATH redirected to libGL without glvnd.

The driver is Mesa/radeonsi, through presumably all drivers are affected because the extra CPU time is spent in glvnd, not Mesa.