Yes that's the same corruption I have with the native backend. I suggest you to
have a look at top: native isn't slow, but aftware a few minutes the X process
starts eating the CPU ,that's the reason ;)
On the contrary Raster in EXTREMELY slow since the very first moment, but at
least there is no corruption.

Regarding your glxgears issue "vblank_mode=0 glxgears" will do the magic.