Google updated Lighthouse 8.0 and one of the important changes is to change the weights of the different speed metrics. The changes are estimated to make it easier for most sites to achieve higher page speed scores. Cumulative Layout Shift scores will likely trend higher while other metrics become stricter.
Google Lighthouse 8.0
Lighthouse is a tool that helps publishers, developers and SEOs measure page speed and other metrics related to accessibility and SEO. Lighthouse powers the online PageSpeed Insights tool and is shipped as a developer tool in Chrome.
The new Lighthouse version 8.0 will ship in Chrome 93 but is available right now in PageSpeed Insights.
Changes to Google Lighthouse Tool
- Performance score has been re-weighted
- Total Blocking Time (TBT) Scoring is Stricter
- First Contentful Paint (FCP) Scoring is Stricter
- Cumulative Layout Shift (CLS) Scoring is Less Strict
How Scores Have Changed
According to Google’s FAQ, about 20% of sites will see a drop of up to five points, while 20% might not see much change.
Google estimates that approximately 60% of sites will see a positive change, from moderate to 5 points improvement or higher.
There are estimated to be more winners with Lighthouse 8.0 that score higher than sites that score less well.
Here are Google’s estimates on expected changes and explanation:
“~20% of sites may see a drop of up to 5 points, though likely less
~20% of sites will see little detectable change
~30% of sites should see a moderate improvement of a few points
~30% could see a significant improvement of 5 points or more
The biggest drops in scores are due to TBT scoring becoming stricter and the metric’s slightly higher weight. The biggest improvements in scores are also due to TBT changes in the long tail and the windowing of CLS, and both metrics’ higher weights.”
Total Blocking Time (TBT) Stricter Scoring
TBT is a measurement of how long it takes for a web page to become responsive to scrolling or keyboard taps. The goal is to have a low TBT score which corresponds to a page that is nearly immediately interactive.
The newly updated Total Blocking Time scoring is described as being stricter.
According to the documentation the scoring could have been even more strict. But the engineers decided to dial it back so as to not be too “jarring.”
According to the TBT GitHub page:
“But we think these as our control points would be too jarring and aggressive for now. Still, there’s room to improve, so we’re doing a small shift of TBT being scored more strictly.”
Before and After Lighthouse Scores
Google published a Lighthouse Scoring Calculator showing what the differences are between versions 8.0, 6 and 7 and version 5.
Below are screenshots comparing the exact same Lighthouse scores between the different versions (6/7 and 8.0).
In the example below it shows that version 8.0 results in a lower score.
Below are a comparison of the same Lighthouse scores between versions showing a four point drop between versions 8.0 and 6/7.
Scores Before Update
Scores After Update
Lighthouse API Update
Google published a note about changes to the API:
“The new Cumulative Layout Shift definition is now the default metric surfaced as cumulative_layout_shift, the previous Cumulative Layout Shift metric will be available for a limited time as it is phased out as experimental_uncapped_cumulative_layout_shift.
Largest Contentful Paint has undergone adjustments in recent Chrome versions and has been updated similarly in CrUX.
First Contentful Paint tri-binning thresholds have been updated to be: [0-1.8s], (1.8s-3s), [3s-∞].”
Check Out the New Page Speed Scores
The new scores are available immediately at the PageSpeed Insights tool.
Lighthouse 8.0 will be available in Chrome 93 which is currently scheduled to ship on August 31, 2021.
Lighthouse 8.0 Performance FAQ
GitHub page for CLS
core: update cumulative-layout-shift #12554
GitHube Page for Total Blocking Time (TBT)
core(scoring): update TBT score curve #12576
Web.dev Total Blocking Time Explainer
Lighthouse API Changelog Notes