Hey folks! SpeedCurve contributed hero element times to WebPageTest a little while ago, although they’re currently only available behind a feature flag: https://www.webpagetest.org/?heroElementTimes=1 - the reason it’s behind a feature flag is because the WebPageTest implementation differs slightly to our original implementation in SpeedCurve, and we wanted to iron out any issues first. We’re pretty confident that it’s okay now and I’ll be submitting a patch to enable it by default in the coming days.
Besides automatically finding render times for the largest heading, image, and background image; WebPageTest will automatically find render times for any element with the elementtiming attribute. It also lets you specify your own element selectors: https://github.com/WPO-Foundation/webpagetest/blob/master/docs/Metrics/HeroElements.md
The major drawback to how we’ve implemented hero rendering times is that we rely on the filmstrip to perform a frame-by-frame analysis. For this reason, we can only detect rendering times for elements that at least partly visible in the viewport.