Normally there shouldn't be any delay between postbacks. Does the delay occur on the client side (browser "hangs" for some time and then postbacks) or on the server side (page postbacks but it takes lots of time to load it back)? How many nodes does your treeview have? Also are you using load on demand and PersistLoadOnDemandNodes="true"?
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.