Everyone else in the universe has probably already figured this out, but in case they haven't...
I'm currently consulting on a farily large C# project using Visual Studio 2005. By fairly large, I mean about 27 projects in the main solution.
There are over one hundred custom controls contained across these projects. When the application is built and I go to a design surface, VS2005 likes to update the toolbox with every possible drag-and-drop control it can find. As you might imagine, this takes a very long time complete, even on my 3.6GHz dual-core HT processor (Intel 840 EE).
Well, I finally figured out how to stop it from happening: From the IDE's main menu, click Tools -> Options -> Windows Forms Designer -> AutoToolboxPopulate = False . Additionally, there's an option there to turn off the refactor when renaming a control, which also used to piss me off to no end.
So there. You probably already knew this. And if you did, well, screw you for not telling me.