If you have multiple update panels in web page, make sure that you put updatemode for update panel to ”Conditional” to prevent loading of other update panels in any asynchronous post back.
By default update panel will have the updatemode property to always means if any asynchronous pushback is done for any of the update panel it loads all the update panels in the page. This is against to partial post back rendering.
So it is always a best practice to set updatemode property to conditional to optimize update panel performance wise and to load best results.
To observe this observe Trace the Requests and Responses using httpwatchpreofessional 5.2