Test Load Time of iframe

I have a webpage that I am currently using WPT to get the first and repeat view times of page load. What I am looking to do is further test the time it takes for a specific iframe to fully load on the web page. Is that possible? I seem to be blocked on expanding my WPT test to monitor other aspects besides the document.ready element.

Note: Running WPT as a nodeunit client.

Not directly but if the iFrame is same-origin yu might be able to reach into it and query the navigation timing times from the frame itself using javascript and run the script as a custom metric at the end of a test.