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New performance metrics, updates to PageSpeed ​​Insights and Chrome User Experience Report (CrUX), and more.

At the Chrome Developer Summit, Paul Irish and I announced updates to Lighthouse - Lighthouse CI, a new performance scoring formula, and more. Along with the great news from Lighthouse, we introduce exciting performance tool developments including new performance metrics, updates to PageSpeed ​​Insights and Chrome User Experience Report (CrUX), and insights from the Web Almanac analytics of the web ecosystem.

New performance metrics

Measuring the nuances of a user's experience is the key to quantifying the impact it has on your results and tracking improvements and regressions. Over time, new metrics have evolved to capture those nuances and fill in the gaps in user experience measurement. The latest addition to the metrics story are two field metrics - Largest Content Paint (LCP) and Cumulative Design Change (CLS), which are being incubated in the W3C Web Performance Working Group, and a new metric. Laboratory: Total Block Time (TBT).

Larger Content Paint (LCP)

Largest Content Paint (LCP) reports when the largest content item becomes visible in the viewport.

Before painting with larger content, First Significant Paint (FMP) and Speed Index (SI) were used to capture the loading experience after initial painting, but these metrics are complex and often do not identify when the main content of the page has been loaded. Research has shown that simply looking at when the largest element on the page is rendered best represents when the main content of a page loads.

The new Paint with Larger Content metric will soon be available in Lighthouse reports, and in the meantime, you can measure LCP in JavaScript.

Total Block Time (TBT)

The Total Blocking Time (TBT) metric measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked long enough to prevent input response.

A task is considered long if it runs on the main thread for more than 50 milliseconds. Any millisecond above that is counted towards the lock time for that task.


The total lock time for a page is the sum of the lock times for all long tasks that occurred between FCP and TTI.


While Time to Interactive does a good job of identifying when the main thread calms down later in the load, the total lock time is intended to quantify how tight the main thread is during the load. In this way, TTI and TBT complement each other and provide balance.

Cumulative Design Change (CLS)

Cumulative Design Change (CLS) measures the visual stability of a page and quantifies how often users experience unexpected design changes. Unexpected content movement can be very frustrating, and this new metric helps you address that problem by measuring how often it occurs for your users.

A screencast illustrating how design instability can negatively affect users.

See the detailed Cumulative Design Change guide for how it is calculated and how to measure it.

Lighthouse's new performance scoring formula will soon downplay FMP and FCI and include the three new metrics, LCP, TBT, and CLS, as they better capture when a page feels usable.


See the Lighthouse performance score and the new collection of metrics from for more information.

Field Data Thresholds (CrUX) Adjusted in PageSpeed ​​Insights

For the past year, we have been analyzing web performance from the field through Chrome user experience
(CrUX) data. Using the statistics from that data, we reevaluated the thresholds we use to label a website as "slow," "moderate," or "fast" in field performance.

The term "average" that used to describe sites that fall between "slow" and "fast" is now changed to "moderate", which is more appropriate since this middle group was not related to a statistical average.


To get a general evaluation of a site, PageSpeed ​​Insights (PSI) uses a certain percentile of the total field data distribution as the gold number for that site; The previous thresholds used were the 90th percentile for the first painting with content and the 95th percentile for the first-entry delay (FID).

For example, if a site has a FCP distribution of fast 50%, moderate 30%, slow 20%, the FCP 90th percentile is in the slow section, making the site's overall field score slow.

This has been adjusted to have a better overall layout on the websites and the new breakdown is:

Metric General percentile Fast (ms) Moderate (ms) Slow (ms)
FCP 75th percentile 1000 1000-3000 3000+
DEFENDER 95th percentile 100 100-300 300+

For example, now if a site has a FCP distribution of 50% fast, 30% moderate, 20% slow, the FCP 75th percentile is in the moderate section, making the site's overall field score moderate.

Canonical URL Redirects in PageSpeed ​​Insights

To allow you to measure the user experience as accurately as possible, the PageSpeed ​​Insights team has added a reanalysis message to PSI. For sites that redirect to a new URL, you are prompted to rerun the report on the destination URL to get a more complete picture of their actual performance.


CrUX in the new search console speed report

Search Console launched its new speed report
a week before Chrome Dev Summit. It uses data from the Chrome User Experience Report to help site owners discover potential user experience issues. The speed report automatically assigns groups of similar URLs into "Fast", "Moderate" and "Slow" groups, and helps prioritize performance improvements for specific problems.


Web Almanac


In the opening speech we announced the launch of the Web Almanac, an annual project that combines statistics and trends on the state of the web with the experience of the web community. 85 contributors, comprised of Chrome developers and the web community, have volunteered to work on the project, which looks at 20 core issues about the web that address how sites are built, delivered, and experienced. Start exploring the Web Almanac to learn more about the status of your performance, JavaScriptand third code on the web.

Learn more

For more details on the Chrome Developer Summit performance tool updates, watch the speed tools evolution talk:

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