ATi HD4570 unusable 2D speed


I’m trying to get opensuse 11.1 to work on my laptop (dell studio 1555). So far, I solved sound issue thanks to Your forum and are trying to solve wireless issue.

After some searching and exploring, I installed ati drivers as told here SUSE/openSUSE - . Firstly I updated/installed every dependency and then did every step from that last long how-to.
Now I can finally use native screen resolution (1336768 instead of 1024768), but scrolling in firefox, or moving any window is really, really slow. Almost unusable.
So I read this ATI/Troubleshooting - openSUSE and tried adding that line to my xorg conf, but still the same issue.
Have someone ever encountered this kind of issue? Do you know how to fix it please?

Thank you in advance