Today the .NET team is releasing a cumulative update patch. This has all of the QFEs up until a couple of months ago rolled into one patch. Included as part of that is a patch for WPF that improves the performance of the Windows Workflow Designer as well as a hang that a number of folks have hit. I had a few customers try it out, and they were happy with the improvements. I recommend this update to you if you work with the WF Designer (e.g., editing the workflow for Team Build definitions). There are still perf issues even with this fix, and the WF Designer team has made some very good perf improvements for the next release.
Here is the download page: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=3556.
P.S. This is completely unrelated to the TFS cumulative update that Brian has written about here. I recommend both.