I reassure you the game is absolutely not slow.
@ marcos1880 :
For the screenshot ; it was exactly my problem
I haven't download this drivers because this function is integrated in ubuntu ; after my first installation I had the choice to use proprietary drivers of Ati. I thought it was the best solution to have better performances but it was wrong.
@ divVerent :
flo~$ grep -i driver /etc/X11/xorg.conf
Driver "kbd"
Driver "mouse"
Driver "synaptics"
Driver "wacom"
Driver "wacom"
Driver "wacom"
Driver "ati"