Tại sao primusrun chậm hơn optirun?

Ai đó có thể vui lòng giải thích những kết quả này? Là primusrunquá thông minh cho glxspheresvà chỉ đơn giản là không làm thêm?

...:~$ glxspheres
Polygons in scene: 62464
Visual ID of window: 0xaf
Context is Direct
OpenGL Renderer: Mesa DRI Intel(R) Ivybridge Mobile 
59.811274 frames/sec - 66.749382 Mpixels/sec
59.577921 frames/sec - 66.488960 Mpixels/sec
59.791245 frames/sec - 66.727029 Mpixels/sec
59.723333 frames/sec - 66.651240 Mpixels/sec
...:~$ optirun glxspheres
Polygons in scene: 62464
Visual ID of window: 0x21
Context is Direct
OpenGL Renderer: Quadro K1000M/PCIe/SSE2
190.833067 frames/sec - 212.969703 Mpixels/sec
209.895160 frames/sec - 234.242999 Mpixels/sec
207.980413 frames/sec - 232.106141 Mpixels/sec
...:~$ primusrun glxspheres
Polygons in scene: 62464
Visual ID of window: 0xaf
Context is Direct
OpenGL Renderer: Quadro K1000M/PCIe/SSE2
61.001631 frames/sec - 68.077820 Mpixels/sec
59.677170 frames/sec - 66.599722 Mpixels/sec
59.715808 frames/sec - 66.642842 Mpixels/sec

Ubuntu 12.04 x64, Lenovo W530 w / Intel i7 @ 2.8 GHz

Từ Primus FAQ :

Q: Performance does not exceed 60 fps, I was getting more with optirun/VirtualGL.
A: This is the effect of vblank synchronisation. For benchmarking, you can use 
   vblank_mode=0 primusrun ..., but in practice this will probably only 
   waste power, as your LCD panel does not display more than 60 frames 
   per second anyway.