A rather crude (but nonetheless useful) technique I've used in the past to help isolate browser processing time issues from server side processing times is to stick a couple of pieces of javascript into the top and bottom of the page.
For example, if you can edit your masterpage, at the very top, just inside the HEAD tag, you could put the following.
<script language="javascript">
var xStart = (new Date()).getTime();
</script>
Then and the very bottom of the masterpage, just before the closing BODY tag, add
<script language="javascript">
function ShowPageTimer() {
var xEnd = (new Date()).getTime() - xStart;
alert("Time to load in browser (ms):" + xEnd);
}
_spBodyOnLoadFunctionNames.push("ShowPageTimer");
</script>