Are you running over WIFI? Though actually...the posters should be stored locally right?
Have you tried bringing this up over at the XBMC forums? I honestly can't believe its a graphics drivers. I mean, if the posters were corrupt or not showing up at all then maybe that would be a different story, but them showing up 'slow' - I wouldn't immediately jump onto the graphics drivers.
Do you think the posters taking a while to load up is what is causing the scrolling to be slow? Are you running a mysql database or local database? If this was me, I'd rule out the possibility of the SSD by installing XBMC onto a USB stick and running it in portable mode and seeing if I still had slow scrolling. I dunno, again just I can't see how or why it could be graphics driver related - have you ruled everything else out :S? It sounds like it should definitely be something related to your HTPC and not XBMC but I'm just not convinced of graphic drivers unless you can say you've ruled everything else / mostly everything else out first.
It would be interesting to see what people thought about this over at the XBMC forums, as I liked to know if anyone else thinks this could be graphic driver related / also had similar problems with graphics drivers.