Performance tooling gets new metrics and sharper field data

Chrome Developer Summit 2019 brought a wave of updates for developers trying to quantify user experience. Alongside the Lighthouse announcements—CI support, a new performance score formula, and more—the Chrome team detailed fresh performance metrics and significant changes to PageSpeed Insights and the Chrome User Experience Report (CrUX). Here's what changed and why it matters.

Three new metrics to track

To better capture what a user actually experiences, two new field metrics and one lab metric are being introduced. The field metrics, Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS), are now being incubated in the W3C Web Performance Working Group. The new lab metric, Total Blocking Time (TBT), is ready for use in testing environments.

Largest Contentful Paint (LCP)

LCP reports the moment when the largest content element becomes visible in the viewport. It's designed to address shortcomings in older metrics like First Meaningful Paint (FMP) and Speed Index (SI), both of which were complex and often failed to identify when the main content of a page had actually loaded. Research indicates that the render time of the largest page element correlates better with when users perceive the main content as ready.

While LCP will soon appear in Lighthouse reports, developers can measure it today directly in JavaScript.

Total Blocking Time (TBT)

TBT measures the sum of all time between First Contentful Paint (FCP) and Time to Interactive (TTI) during which the main thread was blocked long enough to hinder input responsiveness. A task is considered "long" if it occupies the main thread for more than 50 milliseconds; every millisecond beyond that 50 ms threshold counts toward blocking time.

A diagram representing a 150 millisecond task which has 100 miliseconds of blocking time.

TBT is the aggregate of the blocking times for every long task occurring between FCP and TTI. Where TTI identifies when the main thread calms down late in the load process, TBT quantifies how strained the main thread is throughout the entire load. The two metrics work together for a fuller picture.

A diagram representing a five tasks with 60 miliseconds of total blocking time out of 270 milliseconds of main thread time.

Cumulative Layout Shift (CLS)

CLS quantifies the visual stability of a page by measuring how often users encounter unexpected layout shifts. Shifting content is a common source of frustration, and this metric identifies how frequently it occurs for real users.

A screencast illustrating how layout instability can negatively affect users.

For the mechanics of calculation and measurement, the detailed CLS guide covers the specifics. The upcoming Lighthouse performance score will rebalance its formula: FMP and FCI will be de-emphasized, while LCP, TBT, and CLS will be added to reflect when a page feels genuinely usable.

In Lighthouse v6 First Contentful Paint, Speed Index, and Largest Contentful Paint are the main load performance metrics; Time To Interactive, First Input Delay, Max Potential First Input Delay, and Total Blocking Time are the main interactivity metrics; And Cumulative Layout Shift is the main predictability metric.

PageSpeed Insights recalibrates field data thresholds

After a year of analyzing web performance through CrUX field data, the Chrome team has reassessed how PageSpeed Insights (PSI) labels sites as "slow," "moderate," or "fast." Previous assessments relied on specific percentiles of user experience data—90th percentile for First Contentful Paint and 95th percentile for First Input Delay (FID)—to assign an overall score. This had a skewing effect: a site with 50% fast, 30% moderate, and 20% slow FCP results would land in the "slow" bucket because the 90th percentile is in the slow range.

In response, the thresholds have been adjusted for a better distribution across websites.

Metric Overall Percentile Fast (ms) Moderate (ms) Slow (ms)
FCP 75th percentile 1000 1000-3000 3000+
FID 95th percentile 100 100-300 300+

The same example now illustrates the change: with a distribution of 50% fast, 30% moderate, and 20% slow, the FIFTYFIFTH percentile FCP falls in the moderate range, making the overall score "moderate."

PSI handles redirects and aligns with Search Console

Two additional updates improve how PSI reflects real user experiences. First, for sites that redirect to a new URL, PSI now prompts a reanalysis on the canonical landing URL, giving a more complete view of actual performance.

PSI user interface showing the URL redirect and the 'Reanalyze' button

Second, the new Search Console Speed report launched ahead of the conference, using CrUX data to help site owners surface experience problems. The report auto-groups similar URLs into "Fast," "Moderate," and "Slow" categories, and directs attention to the most impactful fixes for specific issues.

Search Console Speed report.

Web Almanac: a snapshot of the web ecosystem

The opening keynote announced the Web Almanac, an annual project combining statistics and trends about the web with community expertise. Contributors—85 Chrome developers and members of the web community—analyzed 20 core areas covering how sites are built, delivered, and experienced. Early topics include performance, JavaScript, and third-party code.