I'm having some huge slowdowns when I have some XAML open. I had the problem in 5.0 and its still around in 5.1 (Build 5.1.1707.19 on 2010-06-09T22:40:28). I've uploaded a profile (profile 2.7z to your ftp site/.upload) using dotTrace while I was getting freezes of several seconds while trying to navigate around an XAML file. The performance problems go away when I disable R#.
I'm hoping that the profiles might expose the problem I'm having so that it can be fixed or maybe there's some setting I can disable to fix the performance problems. I love using R# but the freezes are infuriating ;)