Is there a JavaScript library that can perform timing of the webpage when it loads? For example the customer calls with a performance problem. We tell them to turn on a "debug" checkbox, which would send browser performance info back to the web-server, and the web-server would then phone home. We can then see exactly what was happening in their browser.

I'm interested in something like what FireBug "Net" tab or YSlow produces, without requiring the customer to install anything or modify their browser in any way. Does it exist (commercial software is OK)?