First of all, it looks like you're creating a separate information (or visualization) layer for every item in your collection (you wrote that you use radMap.ItemSource = myCollection). You definitely shouldn't use this approach as you don't need a separate layer for every item. You need just 1 layer for all your items.
Unfortunately from your description I cannot tell for sure what kind of items you display (simple framework elements or map shapes) but in any case please note that 1 million items is too much for the WPF rendering engine. Therefore I would recommend using the items virtualization and items clustering features available with the VisualizationLayer
object (it is replacement for the old InformationLayer
object). You can find such examples here:
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