If you feel that Visual Studio 2010 UML or class diagramm editor became slow and not as responsive as it is used to me, especially if you are working with many objects at a time, then you should try to close the property window. Strangely enough this window gets populated every time you select an object and in case if you select many objects it shows only the commonly set properties. This is the reason why the editor becomes so slowly (even though iit does not explain why it takes so long to populate the properties window).