It's seems that it's a problem of the intel video driver. More the problem is caused by EXA acceleration that lack in performance, take a look to https://bugs.launchpad.net/xserver-xorg-video-intel/+bug/177492 unfortunaly i've tried every solution mentioned in that thread without luck, for now i've disabled EXA in flavor of XAA but it's a bit bugged. bye, Dino