衡量客户端页面加载时间的最佳方法是什么？

I'm looking to monitor the end user experience of our website and link that with timing information already logged on the server side. My assumption is that this will require javascript to to capture time stamps at the start of request (window.onbeforeunload) and at the end of loading (window.onload). Basically this - "I'm looking to monitor the end user experience