According to Matt Winkle some performance enhancements have been made to the Workflow designer in Visual Studio 2008 SP1. Strangely enough no mention of these improvements is made in the release notes. I wonder if they made it in.

Matt Winkle: "In .NET 3.5 SP1, there are no features introduced, but there have been a number of internal improvements made, including some substantial perf gains in the WF designer (for certain scenarios)."