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Core Web Vitals
WebPageTest's Web Vitals Diagnostics shows you how your Core Web Vitals metrics compare to the field, where you need to optimize, and allows you to drill deeper into your page load.
Loading
Largest Contentful Paint (LCP)
A fast LCP delights your users with snappy experience, so it's important you ensure the main content loads quickly.

WebPageTest shows you when and where LCP fires in your page load and highlights where you can optimize.
visual stability
Cumulative Layout Shift (CLS)
Unexpected layout shifts can be disruptive to users, so a low CLS helps ensure your page is delivering a quality experience.

WebPageTest focuses on your site's 'windows' of layout shifts and highlights the visible elements impacting your CLS score.
interactivity & REsponsiveness
Total Blocking Time (TBT)
Poor interactivity and responsiveness disrupts your user's initial experience, so a good First Input Delay (FID) or Total Blocking Time (TBT) is important to making a good first impression.

WebPageTest uses the lab metric Total Blocking Time to measure load responsiveness and highlights the waterfall view of blocking elements.
"We improved our CLS by  ▲ 250%  with help from WebPageTest's 'Highlight Layout Shift'."
Chris Boakes, Developer, The Telegraph
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