What are you trying to measure - the load time on a single PC e.g. developer environment or the load time for a whole bunch of users?
If it's a single PC you might want to look at something like DynaTrace AJAX Edition (http://ajax.dynatrace.com/ajax/en/) or HTTPWatch (http://www.httpwatch.com/)
If it's for a large groups of users a Real User Monitoring based approach could fit what you want - if you want to roll your own look at boomerang.js (it's on GitHub) or for off the shelf NewRelic
Other option so sort of Real User Monitor is to add the tracking of page load times to Google Analytics if you use, Just add _gaq.push(['_trackPageLoadTime']);
to your tracking code.