Telerik RadTreeView does support a large number of nodes, however it can only show a limited number of nodes at a time on client side.
Imagine 10,000 lines of text and values (say, 100 bytes per line) - this alone makes 1MB HTML size, which actually makes showing 10,000 lines of text at the same time in a web-context virtually impossible. That's why large datagrids use paging for showing data.
Basically a tree with more than 200 nodes should not be displayed at once without Load On Demand. You can test this with any other treeview products - you will have results similar to ours.
For details on the matter please refer to the control's documentation under section Controls --> RadTreeView --> Load On Demand Support
the Telerik team