I upgraded my notebook to Ubuntu Karmic Koala 9.10. As known from the other upgrade before, here was the same problem with the slow xorg. So I tried the other solution posted below but that won't work in this case. So I searched the Ubuntu Forum again and found the solution.

You just have to download the new driver posted in the link below, it's a beta version i know, but it works and install the driver, add in the following in the xorg.conf (/etc/X11/xorg.conf) file and reboot the system and enjoy the working X.

Add or if exist overwrite this in the Xorg.conf
Driver "sis671"

Source:
www.ubuntuforums.org