VS 2010 - How To Determine Why Site Takes 5 Minutes Partial Postback

I have a web application which has rather a lot of controls. I also have several buttons (add, edit, save, cancel). Most of my content page is in an update panel so it does not process the whole page. On form submit I have some javascript code which shows a div 50% opacity and an animated gif to prevent users clicking on the buttons more than once whilst it is processing the data to the server. Here is the javascript / call:

Code:
onsubmit="overlay();"
function overlay() {
el = document.getElementById('overlay');
el.style.display = "inline";
document.getElementById('Image2').src = "../Images/loader.gif";
}
function removeoverlay() {
el = document.getElementById('overlay');
el.style.display = "none";
}

The problem is it the form disables like I want it and the giff animates for about 1.5 seconds but it does not enable the form for a further 4 seconds.

Is there anyway to see what it happpening at this point. If I put some watches in my code behind it runs through less than a second so what else can it be doing.

Also on some laptops the form does not fit vertically so there is a scroll bar. When the div is shown on a submit it does not do the full screen including the scroll area.