How it works
Largest Contentful Paint (LCP) measures when the largest visible element appears, usually a hero image or block of text; up to 2.5 seconds is good, over 4 seconds is poor. Interaction to Next Paint (INP) measures how quickly the page visibly responds to clicks, taps and key presses; up to 200 milliseconds is good, over 500 is poor. Cumulative Layout Shift (CLS) scores how much content jumps around while loading; up to 0.1 is good, over 0.25 is poor.
Google rates the numbers from real Chrome users in the Chrome UX Report (field data) over 28 days, at the 75th percentile of page loads. A page only passes when all three metrics are good. INP replaced the older First Input Delay (FID) in March 2024.
Lab tools such as Lighthouse in PageSpeed Insights simulate a page load without real interaction, so instead of INP they measure Total Blocking Time (TBT): how long scripts block the main thread. TBT is not a Core Web Vital, but it tracks closely with INP.
Why it matters
Slow or jumpy pages cost visitors and revenue: people who wait too long for the hero image or tap a button that just shifted out from under them leave. Core Web Vitals are also part of Google Search’s page experience signals, so when content is otherwise comparable, poor numbers can hurt rankings.
The numbers usually degrade gradually through a new slider plugin, unoptimized images, a late-loading consent banner or extra tracking scripts. Without regular measurement, nobody notices until rankings or orders drop.
How to prevent it
- Serve the hero image at the right size in modern formats such as WebP or AVIF, do not lazy-load it, and give it fetchpriority="high".
- Give images, videos, ads and iframes fixed dimensions or an aspect-ratio so nothing shifts, including banners injected at the top of the page.
- Cut down on JavaScript: remove unused plugins, load third-party scripts with defer or only after interaction, and break up long tasks.
- Use caching, Brotli or gzip compression and a CDN where it helps, so the server’s first response arrives quickly.
- Measure on mobile before and after every major release, and keep an eye on the field data in Search Console’s Core Web Vitals report.
How DomainWarn detects it
The PageSpeed check measures the homepage weekly through PageSpeed Insights, rating the mobile run and recording desktop for the history: the Lighthouse score with LCP, CLS and blocking time (TBT) and, where Google sees enough real users, the LCP, INP and CLS field data from the Chrome UX Report. A score below 50 or poor field data is a warning and opens the incident “Website loads slowly (PageSpeed)”; missing field data is noted as information.
The Page health check measures LCP, CLS and Total Blocking Time during the daily browser visit from a location in Germany. A value above the “poor” threshold (4 seconds, 0.25 or 600 milliseconds respectively) is a warning, and DomainWarn only opens the incident “Web Vitals persistently poor” after three runs in a row, so measurement noise does not trigger alerts. DomainWarn does not measure INP itself; it only reports it from Google’s field data.
DomainWarn checks this for all your domains on a schedule and alerts you when something is missing or changes.