Just a short update of my current situation: By deinstalling Expression Web from my Vista machine I finally also got rid of the several-minute delay when opening the designer or split view for the first time. So the Visual Web designer is mostly usable for me now.
BUT (and I'm afraid it's still a big but and a major issue for me that hopefully also will be addressed in SP1):
A problem that still has not gone away is that if I copy some text in the source or split view from the markup I get the busy cursor and everything stops for up to 2-3 minutes. The really bizarre thing in my eyes is that it does not happen when trying to paste something but when just copying something to the clipboard. This makes working with the Visual Web Designer still somewhat tedious and slow. Any ideas regarding that ?