Hi Paul,
Unfortunately you won't be able to do that. You can AJAX load all child nodes of a tree node, but all child nodes of a single node are loaded during one call. Generally having more than 5000 items on a single node should be avoided at design level for two reasons:
1. Performance issues as you have noticed. No TreeView in the world can have 5000 items on a single node and still runs smoothly;
2. Usability issue. Your user will have a hard time to find what they want in that TreeView.
Thus you should considering reorganize your data to split it into small categories, (for example, if the items are "Client Names", you can split them into "A" , "B", ..."Z" based on first letter). Once you split them, you can then use it together with popuplate on demand feature to AJAX load a node at a time. Here is an example of this feature:
http://www.essentialobjects.com/Demo/Default.aspx?path=TreeView\_i1\_i1Thanks