I can't say exactly about a reason of the freezing of the UI without your solution. But I don't think that the bing provider assignment could be a cause of it. It can occur when you assign your polygon list to a property of ViewModel. I think that the property is bound to the ItemsSource property of an information layer. In this case the layer should process the polygons in the list. So, the processing of them will require a time depending of the length of list.
All the best,
the Telerik team
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