Microsoft has released a Hotfix for the incredibly annoying performance issues when working in the Design view of an Asp.Net form that contained custom user controls.
The installation info is on the MSDN Blog and is pretty self explanatory. It takes a few minutes to install but it is worth the wait. Before the patch, simply switching to Design view for one of my Asp.Net forms could take upwards of 30 seconds. After the patch, it takes around 3 seconds and half of that is the pulling together the nested master pages behind the form.
I've also noticed that the properties window now switches almost immediately when I click on a control. Before, it would take 5-10 seconds to change and sometimes never did.