Actually the tree view internally stores some of the selected items (not all). If huge amount of records are selected then there could be fair amount of selected items stored internally in order to maintain certain functionality. Also, selected items require additional processing when they enter or leave the visual area of the screen. So, bigger visible area implies more selected items to be rendered. This could bring to performance issues while scrolling. Do you really need to select so many items?
the Telerik team
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