Then, I started a little investigation trying to figure out what was going on, and I realised that most CPU time was spent by Xorg server. So, I suspected that something in the graphic acceleration was happening.
After some time googling, I read that intel graphics driver has been actively modified in order to introduce better acceleration and memory management code, these modifications has implied to remove the legacy acceleration code.
Unfortunately these new features are not enabled by default because of stability.
Enabling the new and faster acceleration method (UXA) my laptop increases the performance considerably and I've not released any problem.
So if you have an Intel integrated graphics chip, add this line to the Device section in your /etc/X11/xorg.conf file:
Section "Device"
[...]
Option "AccelMethod" "uxa"
EndSection
