Metrics

Metrics in CrUX are powered by standard web platform APIs exposed by browsers. In the BigQuery dataset in particular, this data is aggregated to origin-resolution. Site owners requiring more detailed (e.g. URL-level resolution) analysis and insight into their site performance can use the same APIs to gather detailed real user measurement (RUM) data for their own origins. Note that while all APIs are available in Chrome, other browsers may not support the full set of metrics.

Most metrics are represented as a histogram aggregation, allowing visualization of the distribution and approximation of percentile values.

First Paint

"First Paint reports the time when the browser first rendered after navigation. This excludes the default background paint, but includes non-default background paint. This is the first key moment developers care about in page load - when the browser has started to render the page."

Paint Timing API

First Contentful Paint

"First Contentful Paint reports the time when the browser first rendered any text, image (including background images), non-white canvas or SVG. This includes text with pending webfonts. This is the first time users could start consuming page content."

Paint Timing API

DOM Content Loaded

"The DOMContentLoaded reports the time when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading."

MDN

Largest Contentful Paint

"Largest Contentful Paint (LCP) is an important, user-centric metric for measuring perceived load speed because it marks the point in the page load timeline when the page's main content has likely loaded — a fast LCP helps reassure the user that the page is useful."

web.dev/articles/lcp

Onload

"The load event is fired when the page and its dependent resources have finished loading."

MDN

Cumulative Layout Shift

"Cumulative Layout Shift (CLS) is an important, user-centric metric for measuring visual stability because it helps quantify how often users experience unexpected layout shifts — a low CLS helps ensure that the page is delightful."

web.dev/articles/cls

First Input Delay

"First Input Delay (FID) is an important, user-centric metric for measuring load responsiveness because it quantifies the experience users feel when trying to interact with unresponsive pages—a low FID helps ensure that the page is usable."

web.dev/articles/fid

Interaction to Next Paint

"Interaction to Next Paint (INP) is a field metric that assesses responsiveness. INP logs the latency of all interactions throughout the entire page lifecycle. The highest value of those interactions—or close to the highest for pages with many interactions—is recorded as the page's INP. A low INP ensures that the page will be reliably responsive at all times."

web.dev/articles/inp

Interaction to Next Paint (INP) was added to the CrUX dataset in February 2022. This new metric captures the end-to-end latency of individual events and offers a more holistic picture of the overall responsiveness of a page throughout its lifetime.

Experimental metrics

Experimental metrics are available in the CrUX dataset via BigQuery, with some also available in the CrUX API. These metrics are likely to change regularly as they evolve based on user feedback. Check the release notes to keep up to date on the latest changes.

Time to First Byte

TTFB is only collected on full page loads, unlike other timers (such as LCP) which are also collected on back-forward navigations and pre-rendering. As such, the sample size of TTFB can be smaller than other metrics and may not necessarily be compared directly with them.

Popularity

The popularity rank metric is a relative measure of site popularity within the CrUX dataset, measured by the total number of navigations on the origin. Rank is on a log10 scale with half steps (e.g. top 1k, top 5k, top 10k, top 50k, top 100k, top 500k, top 1M, etc.) with each rank excluding the previous (e.g. top 5k is actually 4k URLs, excluding top 1k). The upper limit is dynamic as the dataset grows.

Popularity is provided as a guide for broad analysis, e.g. to determine performance by country for the top 1,000 origins.

Notification Permissions

For websites that request permission to show users notifications, this metric represents the relative frequency of users' responses to the prompts: accept, deny, ignore, or dismiss.