Core Web Vitals
Definition
The three cover loading, responsiveness and visual stability in turn. Largest Contentful Paint records when the biggest visible element has finished rendering — good is 2.5 seconds or under, needs improvement runs to 4 seconds, and anything slower is poor. Interaction to Next Paint measures the worst delay between a tap or click and the screen updating in response — good is 200 ms or under, needs improvement runs to 500 ms, poor is beyond that. Cumulative Layout Shift scores how much content jumps around while loading, on an unbounded scale where 0.1 or less is good and above 0.25 is poor. INP replaced First Input Delay in March 2024. A page passes only if all three land in the good band at the 75th percentile of visits over a rolling 28 days, counted separately for mobile and desktop.
Why It Matters
Because the assessment is at the 75th percentile of real traffic, a site can feel fine on a developer's laptop and still fail — the quarter of visitors on older phones and slower networks are the ones being measured. The ranking effect is genuinely modest, a tie-breaker rather than a lever, but the underlying behaviour is not: a layout that shifts under a reader's thumb makes them tap the wrong link, and a 3 second paint loses a share of visitors before anything appears. Static pages tend to start ahead, because there is no server rendering time and the files can sit on an edge node near the visitor. The common failure is not the hosting at all — it is an oversized hero image, a blocking font, or an advert slot with no reserved height.
How It Works
Two sorts of data exist and they answer different questions. Field data comes from the Chrome User Experience Report, collected from real Chrome users who opted in, and is what Search Console and the ranking signal use; it is a 28-day rolling window, so a fix takes weeks to show. Lab data comes from a tool such as Lighthouse running a single simulated load, which is reproducible and useful for debugging but cannot produce a real INP figure, since nobody clicked anything. In the browser, each metric arrives through a PerformanceObserver — largest-contentful-paint, event and layout-shift entry types — and the web-vitals JavaScript library wraps those into one number per metric. Of the three, Largest Contentful Paint is usually where the work is, and it is the one most directly affected by how the files are delivered.
Real-World Example
A recruiter publishes a hiring page on 99helpers at join-halden.99helpers.site with a 2.4 MB PNG banner. Field data shows LCP at 4.1 seconds on mobile, CLS at 0.28 because a web font swaps in late and reflows the headline, and INP comfortably inside 200 ms since the page barely runs any script. Exporting the banner as a 180 KB WebP and giving the headline element a fixed height brings LCP to 1.6 seconds and CLS to 0.02 — two failures cleared without touching the copy.
Common Mistakes
- ✕Judging a site by its Lighthouse score alone — that is one simulated load, while the ranking signal uses 28 days of real visits
- ✕Fixing a problem and expecting the numbers to move the same day, when the field window takes weeks to catch up
- ✕Optimising the desktop experience while the failing bucket is mobile, which is assessed separately
- ✕Treating a passing average as a pass — the threshold is the 75th percentile, so a good median can still hide a failing quarter of visits
Related Terms
Largest Contentful Paint
Largest Contentful Paint, or LCP, marks the moment the biggest visible element in the viewport has finished rendering. It is the closest single number to the question a visitor actually asks: when did this page look like it had loaded?
Time to First Byte
Time to First Byte, or TTFB, is the gap between a browser starting a request and the first byte of the response body arriving. It measures everything that has to happen before the page can begin — not how long the page takes to finish.
Latency
Latency is the delay between sending something and it arriving — usually quoted as round-trip time, the milliseconds for a packet to reach a server and come back. It is a separate thing from bandwidth, which is how much data fits through per second once the first packet lands.
CDN
A content delivery network: a set of servers spread across the world that keep copies of your files and answer each visitor from somewhere close to them. It is the service; the machines it runs on are its edge network.
Web Analytics
Measuring who visits a site and what they look at. The two methods — a script running in the visitor's browser, or counting each request as it is served — produce different numbers, and neither of them is wrong.
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