FAQ: Toolbox is slow to populate
Q: Whenever I accidentally mouse over the Toolbox it locks up VS for a while. What is going on?
A: This should only happen the first time you mouse over the Toolbox after starting VS. VS is populating the window with the controls that are defined within your solution in addition to the pre-defined controls. This can be a slow process. To speed it up go to ToolsOptions -> Windows Form DesignerGeneral and set AutoToolboxPopulate to false. This will tell VS not to scan your solution looking for controls to add to the toolbox.