After Google's announcement about Lighthouse 8 this past month, we have updated our test agents. We've gotten a lot of questions about what has changed and the impact on your performance metrics, so here's a summary.
In case you missed it, Lighthouse is a speed tool created by the Chrome Developer team at Google. Lighthouse is a score based system that evaluates lab data (synthetic) through a series of audits in order to identify how your application will perform in the wild.
There are five categories evaluated: Performance, Accessibility, Best Practices, SEO, and Progressive Web App (PWA). As part our synthetic testing, we run a separate Lighthouse test that produces a full audit and allows you to track these scores over time, alongside your other favorite performance metrics.
No metrics were added or removed from the Lighthouse scoring in version 8. The biggest changes are:
Not surprisingly, we see a shift in importance for metrics related to Core Web Vitals. Largest Contentful Paint (LCP) remains the heaviest weighted, while Cumulative Layout Shift (CLS) shows the biggest increase.
CLS now uses session windowing to more accurately measure layout shifts for longer-lived sites, including single page applications. While we don't anticipate a major change for CLS scores measured in a lab environment such as Lighthouse, field data (aka real user monitoring) should generally see improvement for those types of sites as it is adopted more broadly.
Google anticipates the majority of sites could see improvements to their scores, based on their analysis the HTTP Archive.
Our own research here at SpeedCurve suggests that there will be a moderate increase of the performance score (1-5 points) for most sites. Sites that had a high CLS score and/or TBT were penalized more heavily due to the weighting changes and saw a decrease in the score which was sometimes significant (5-10 points).
In addition to the performance weighting changes, Lighthouse has added a few pretty cool features to the report:
It can be a bit overwhelming to look through the list of audits in your report. Filtering by the metric you're interested in can help slim down your performance to-do list.
If you are looking for more details around Lighthouse, or specifics around version 8, here are some good resources: