I have a Windows Server 2008 R2 box with the Desktop Experience feature enabled, so I have the Aero Basic theme going (because I find 2008R2 in Classic Mode an even worse experience compared to Vista's).
When I launch a program from its button on the taskbar the explorer process freezes for a few moments and the start button gets a white background.
Here's a screenshot:
After about 20 seconds or so Explorer un-freezes and the program I wanted to run is finally launched.
I've had a look with AutoRuns and I can't find any unusual shell extensions that might be causing this slow-down.