partial page render after page load with out button click (without asynchronous call back)

Hi all , I have requirement of partial page rendering, but there is no buttons to click so I can call asynchronous trigger.

in my web page there is a panel to load with server side script. which make whole page load slow.  I am not sure how to call this server side script with out a button click.

I did few pages with forms including update panel and asynchronous triggers to load part of page. but in this case I need to load the rest of the page after page load complete call this server side script to fill the panel

any ideas please