Hi there,
UltraWeb controls will be retired when 11.1 release goes out. I suggest that you do not use this control for D&D but use the new WebDataTree control which has state of the art D&D working in all browsers with rich client side model. Using WebDataTree D&D you can drag one or more nodes to the same or other trees on the page. It is fast and we have tested it with trees having 6000 of nodes.
Take a look at http://samples.infragistics.com/2010.3/WebFeatureBrowser/Default.aspx -> WebDataTree -> Drag and Drop
Hope this helps.