Update Panel is a great tool for presentation layer but not performance...
An UpdatePanel used this way improves the user experience, but it does little to reduce the volume of data being passed over the wire. The UpdatePanel hardly reduces the load on the server, either—up to the point that the controls inside the UpdatePanel render, the processing performed on the server is almost identical to what happens during a full-blown postback. It has to be this way because one of the benefits of the UpdatePanel control is that server-side event handlers like GetCityAndState work no differently inside an asynchronous callback than they do in a traditional postback. That means controls on the page have to be instantiated, they have to be initialized, they have to have access to view state, and so on.
http://msdn.microsoft.com/msdnmag/issues/07/06/WickedCode/
