There might be something going on with the drivers, maybe they don't like multipass solutions.
Do you have Gallium too...? Sry Linux noob here.
This made me curious and i tested it on a crappy gf 8400.
Code:
xbrz4x: 11 FPS
xbrz6x: 5 FPS
xbrz: 31 FPS
I guess that should do for Intel iGPUs too.
If not, please let me know.