Last time I worked on a high-volume website, we did several things, including:
- We used Yslow to get an analysis of the individual factors affecting page load: https://addons.mozilla.org/en-US/firefox/addon/5369
- performance monitoring using an external, commercial tool called Gomez - http://www.gomez.com/instant-test-pro/
- We stress tested using a continuous integration build, using Apache JMeter. http://jmeter.apache.org/
If you want a quick look, say a first approximation, I'd go with YSlow and see what the major factors affecting page load time in your app are.