Loading the layout of Docking control is not a heavy operation by itself and 10 and more panes should not cause any performance issues. As you noted, there could be heavy UI placed in their content and this slows down the application. However, there is ElementLoaded
event which is fired after a docking element is loaded and this event could help. For example, if can be used for setting a content of the loaded pane, or even start some data operation asynchronously. You can find a sample code here - http://www.telerik.com/help/silverlight/raddocking-save-load-the-content-of-the-panes.html
Hope this information helps.
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