When working on an aspx page that uses nested master pages the features of Design Mode do not work properly and it also causes a significant performance issues. For example when the web form that is using the master page goes into design mode it will not properly show the page contents unless all master pages are also loaded into the open files window. Additially once a web form using a nested master page is loaded in design mode it takes a significantly longer amount of time to save that page to disk after being edited. This occurs even after leaving design mode and also after closing all of the master pages. The only way to fix that is to close the solution and open it again.