AJAX :: All Controls Firing An Event On First Page Postback

I have a web application that shows a page containing between 6 and 20 UpdatePanels, contained with Custom Web (.ascx) Controls.

You can see an example of such a page here: [URL]

The problem I'm getting is that when the user FIRST clicks any of the radio-buttons on any of the controls in the page, ALL the radio buttons that have an active selection ALSO fire an rb_CheckChanged event (see application log below). On subsequent radio-button clicks, only that RB fires the event (as expected).

This behaviour is having a dramatic impact on performance. The page loads quickly, and a "normal" RB click provides an AJAX update correctly within 1 second. The problematic first RB click, however, takes up to SIX seconds to process, which is unacceptable from a user experience standpoint.

Developed in VS.NET 2008, ASP.NET 3.5. Also I converted the "Web Site" solution to a "Web Application" (pre-compiled) and got the same performance (still too slow). Release builds run only marginally faster than debug builds.

Why would a single RB selection change cause all RB's that have a "selection" set on them to also fire? (The "selection" is determined through database settings every time the control is populated, which is called from within the parent page control's Page_Load function. This selection is determined also at first page load, since all the controls get populated then, so why would the clicking of an RB cause them all to fire an event, and only once? That's what I don't get...)

[Code]....