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When you start developing using AJAX ASP .net, on your localhost... everything works magically with good performance, AJAX is quite easy isn't it? Why the people go for more complicated stuff like script services? Just drop a bunch of UpdatePanels and everythings works fine no need to go for more complex code... well that's only true on your localhost :-), once you publish your site on your remote server things change...
By default UpdatePanels get all updated in every request, you can modify this behaviour by setting the UpdateMode to Conditional and you set in your sever side code which update panel gets updated (by doing this you can reduce the size of the response sent to the client).