Ok so I just learned a terrible thing...
It seems that direct rendering is not enabled correctly by XGL at the present time.
For information purpose, I have a nVidia card : 6600, using Ubuntu Dapper with the last 686 kernel and up-to-date drivers and others.
I hope it will be corrected... It's ok because it's UML time, but after that... 5 FPS hurts
Thanks all for your time and help !
Lideln