Core Web Vitals, Explained
A website can appear quickly, respond slowly, or move around while someone is trying to read it. Core Web Vitals put separate measurements around those experiences.
The web.dev guide to Web Vitals groups the current Core Web Vitals around loading, interactivity and visual stability. Keeping those questions separate makes a report more useful: a good result in one area does not explain what happened in another. Start with what the visitor experiences, then use the corresponding measurement to guide a closer look.
Three Measurements, Three Questions
Largest Contentful Paint, or LCP, measures loading performance. The practical question is whether the page's main visual content appears promptly. When reading a report, keep that loading question in view rather than treating the number as a description of everything the page does.
Interaction to Next Paint, or INP, measures interactivity. This brings attention to responsiveness: the experience of doing something on a page and waiting for it to react. A page that has appeared on screen still needs to respond when someone tries to use it.
Cumulative Layout Shift, or CLS, measures visual stability. The question here is whether the layout stays in place. Think about trying to read a paragraph while surrounding content changes position. Stability deserves its own attention alongside loading and responsiveness.
The Thresholds
Web.dev gives these recommended targets for the current set:
- LCP: within 2.5 seconds of the page starting to load.
- INP: 200 milliseconds or less.
- CLS: 0.1 or less.
The same guide says to measure at the 75th percentile of page loads, segmented across mobile and desktop devices. A page passes the assessment when all three measurements meet their recommended targets at that percentile. One strong result does not cancel a result that misses its target.
Keep the units beside the values when taking notes. LCP is expressed in seconds here, INP in milliseconds, and CLS as a score without a time unit. Comparing bare numbers without their labels can obscure which part of the experience needs attention.
Why INP Replaced FID
In its March 12, 2024 announcement, web.dev described INP becoming a stable Core Web Vital and replacing First Input Delay, or FID. The announcement says this addresses shortcomings in the earlier responsiveness measurement. It also states that PageSpeed Insights assesses INP instead of FID in its Core Web Vitals assessment.
This matters when comparing reports that use different metric names. A record labelled FID and a record labelled INP should not be treated as interchangeable simply because both concern responsiveness. Check the label before drawing a conclusion about whether a result improved. The announcement also describes Chrome deprecating support for FID.
Reading a PageSpeed Report
Begin with the metric names, the device category and the percentile being reported. Keep mobile results and desktop results separate, following web.dev's guidance. Write down the result for each of the three measurements so that a single favourable value does not dominate the interpretation.
Then turn each result into a question about the page. For loading, ask what content has to arrive before the page feels ready. For interactivity, ask what happens when a visitor uses the page. For stability, ask what changes position as content appears. These questions provide a useful bridge from a measurement to something that can be inspected.
What Usually Moves the Numbers
Image sizing, space reserved for images or ads, and heavy scripts are sensible places to investigate. An oversized image is a reason to examine loading work. A layout without space for incoming content deserves a stability check. A script-heavy interaction is a reason to inspect responsiveness. Treat these as starting points for investigation, with the page's actual behaviour guiding the work.
After an adjustment, compare the same measurement in the same device category and note what changed. Keep each question tied to its own evidence. The aim is to understand how loading, interactivity and visual stability work together in the experience of using the page.