Thank you for the question.
In your case, you need to redesign your pages so that they don't take so long to load. There is no other way because the XAML loading happens on the UI thread. RadBusyIndicator is also on the UI thread and starting it will have no effect since the thread is busy doing other stuff (loading page content). RadBusyIndicator is appropriate when performing a long running operation like downloading a file from the web or running some algorithm that does real work (calculating subatomic particle collisions for example :) ).
Please write again if you have other questions.
All the best,
the Telerik team
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