I can see in the provided snippets that you actually create UI containers (e.g. RadTreeViewItem). This is the reason why the virtualization cannot take place in your particular scenario. You should create business items and let the control decide when the UI containers should be created.
You can find a sample demonstration how the LoadOnDemand feature can be used with virtualization in our documentation
. Please give this approach a try and let us know if you need any further assistance.
Pavel R. Pavlov
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