When you trigger a full postback on a page, the page stops being updated and until the refreshed page is received from the server, you have no control over it.
To achieve what you are looking for, there are two possible approaches:
1. Use ServerSideCallback mode instead of ServerSide. This way you are still using the same logic to bind the TreeView, with the added benefit of not having to postback the entire page, and the loading icon is showed by default.
2. Put the treeview inside a RadAjaxPanel with a designated RadAjaxLoadingPanel, which will show a loading screen while the postback is completed.
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