I wanted to let you know about a last minute change that went into VS 2010 Beta 2 that you can use to improve the startup performance for the WPF and Silverlight Designer. The change went in late and it was a little risky, so we decided to leave it off until we had a chance to do some more testing with it. You can turn the change on yourself via a registry key. We expect the change will be on all the time in the final product, so changing the registry key is strictly a Beta 2 issue.
You can read more about it here: http://social.msdn.microsoft.com/Forums/en-US/vswpfdesigner/thread/4511d43f-c134-4329-a970-e374252a620e
If you have any feedback on how this works (or doesn't work) for you, please let me know. You can contact me at DevPerf@Microsoft.com.