Does implementing a ICallbackEventHandler gain any performance with respect to ViewState over an UpdatePanel or is it very much the same?
The background is this - I have a MS chart server control on the page and will want to refresh data after a page load. Will an ICallbackEventHandler make it any faster than an UpdatePanel?
Yes, my experience is that ICallbackEventHandler will give better performance than an UpdatePanel. The viewstate will still be posted back as part of the request, but you don't get any updated viewstate returned as part of the response, nor do you have any of the overhead on the clientside of all the Javascript associated with an UpdatePanel. If you have some relatively simple Javascript code to handle the results of your callback, I'd definitely suggest going down that route.