Modulo possible complication due to frames etc, at the very top of the page, such as immediately under the script section within the header.
[tt]
<script type="text/javascript">
var time_s,time_e;
time_s=new Date().getTime();
window.onload=function() {
time_e=new Date().getTime();
alert((time_e-time_s)+" milliseconds");
}
</script>
[/tt]