# Core Web Vitals

> How to check page speed with real-user and lab data, what LCP, INP and CLS mean, and how the weekly check and alerts work.

## What Core Web Vitals are

Core Web Vitals are Google's three measures of how a page feels to a visitor:

| Metric | Measures | Good | Poor |
|---|---|---|---|
| LCP — Largest Contentful Paint | How fast the main content appears | 2.5 s or less | over 4 s |
| INP — Interaction to Next Paint | How fast the page responds to a click or tap | 200 ms or less | over 500 ms |
| CLS — Cumulative Layout Shift | How much the layout jumps while loading | 0.1 or less | over 0.25 |

Between Good and Poor is **Needs improvement**. Google uses these signals in ranking, and slow or jumpy pages lose visitors.

Supporting metrics shown as well: **FCP** (First Contentful Paint — first text or image) and **TTFB** (Time to First Byte — server response time).

## Field data and lab data

Rankfinity shows two kinds of numbers:

- **Field data** — measured from real Chrome users visiting the page over the last 28 days, reported as the 75th percentile (p75): 75% of visits were this fast or faster. Field data is what Google uses. Small sites often have **no field data** because there are not enough visits; the screen says "not measured" in that case, never zero.
- **Lab data** — a test run on demand, on a simulated mobile phone and on desktop. It always exists, so it is useful for diagnosing, but it is one test, not real users. INP needs real interaction, so it is "not measured in a lab test".

## Running a check

1. Open **Site audit › Core Web Vitals**.
2. Click the run button. The check is **free** within daily limits.
3. Rankfinity tests the most important pages from your last crawl (by default five), plus the site as a whole.

A second click within 24 hours shows the stored result rather than testing again. If a daily limit is reached, the screen says when you can run again.

## Reading the results

### The site

Field data for the whole origin (all pages together): p75 LCP, INP and CLS with a Good / Needs improvement / Poor badge, and the share of visits that were good, needing improvement and poor.

### Each page

- **Lab mobile** and **Lab desktop** — LCP, CLS, FCP, TTFB and the overall performance score.
- **Field** — p75 values with badges, if the page has enough traffic.
- **History** — a line of field data for about six months, when available.
- **Top opportunities** — the five biggest suggested improvements with the estimated saving (for example "Serve images in modern formats").

Pages with poor results also appear as a **core_web_vitals** issue in the site audit's Action Plan and in **Copy fix prompt for your AI**.

## Weekly check

Click **Weekly check: on** to test the project automatically once a week. It is free, off by default and uses the same limits. If a limit is reached or there is no crawl yet, it tries again later.

## Alerts on a drop

When a page that was **Good** becomes **Poor** on the same type of measurement (field against field, or mobile lab against mobile lab), you get an alert email together with the site audit alerts. A move from Good to Needs improvement does not send an email.

## In reports and share links

The **Site Audit** report template includes a "Page speed · Core Web Vitals" section, and the client share link shows the same section, using stored results only. See [Client reports and sharing](/docs/reports).

## Improving the numbers

- **LCP:** compress and resize the hero image, serve images in WebP or AVIF, use a faster host or a CDN, avoid render-blocking scripts and fonts.
- **INP:** reduce heavy JavaScript, split long tasks, delay third-party scripts (chat widgets, trackers) until needed.
- **CLS:** give images and embeds a width and height, reserve space for banners and ads, avoid inserting content above what the visitor is reading.
- **TTFB:** enable server caching, check slow plugins or database queries.

Test again after changes. Field data needs up to 28 days to reflect fixes, because it is a rolling window of real visits.

## Common questions

**Why is field data "not measured"?**
The page or site has too few Chrome visitors for Google to publish data. Use lab data until traffic grows.

**Why are lab and field numbers different?**
Lab is one simulated visit; field is thousands of real visits on real devices and networks. Google ranks on field data.

**Does this cost credits?**
No, within the daily limits. The separate **Performance** tab is a paid lab test per URL.

**Can a client run the check?**
Clients can view results. Running checks and the weekly schedule are for the agency.

**Which pages are tested?**
The most important pages from your last crawl, chosen by the same ranking as the Action Plan.