In debugging another problem with TreeView I've found that when a child node of the (hidden) root node is clicked open (to display some grandchildren of root), all nodes that indicated that they have grand children (whether displayed or not and even when never shown before) are queried about their children.

In my case that was only a small number of nodes (30 orso, of which 25 aren't even visible), but I think this could be a major performance issue if those nodes are backed by something with a bit more latency, like say a directory structure on a local filesystem. In fact, even if my case there's some noticable delay (0.5 seconds) when opening the first node like this due to 30 nodes being queried for their children.