On May 26, we updated our Test Agents to the latest version of Lighthouse. Lighthouse 6 comes with a completely revamped Performance Score algorithm, new metrics, audits, and more. All tests from May 26 onwards run using Lighthouse 6, which is why you are likely to observe changes to your Performance Score.
Some Sites, especially ones relying on JavaScript, can observe significant Performance Score changes.
With Lighthouse 6, we are also reporting a new metric—Cumulative Layout Shift. Alongside Total Blocking Time and Largest Contentful Paint (present in Calibre since September and October 2019), CLS is a part of a new generation of performance metrics that aim to portray user experience more accurately.
This also means that you can track all Web Vitals (Total Blocking Time, Largest Contentful Paint and Cumulative Layout Shift) in Calibre. We are committed to keeping our platform on par with the newest developments in the performance space.