Your developer says the site passes Core Web Vitals. Your agency says it does not. Both are looking at different tools measuring different things, and neither has explained what the numbers mean for your business. Here is the version you actually need.
Core Web Vitals are three measurements Google uses to judge page experience: Largest Contentful Paint under 2.5 seconds, Interaction to Next Paint under 200 milliseconds, and Cumulative Layout Shift under 0.1. You need all three to pass, on real visitor data, for 75 percent of visits. They are a ranking factor, but a small one compared with content and links.
What the three numbers measure
Each one maps to a complaint a real visitor would make.
| Metric | Plain English | Passing |
|---|---|---|
| Largest Contentful Paint | How long until the main thing on screen appears | Under 2.5 seconds |
| Interaction to Next Paint | How long the page takes to respond when you tap something | Under 200 milliseconds |
| Cumulative Layout Shift | How much the page jumps around while loading | Under 0.1 |
LCP is the one most sites fail, and it is usually a large hero image or a font that blocks rendering. INP catches pages that look ready but ignore your taps for a second. CLS is the one that makes you tap the wrong button because an ad loaded above your thumb.
Lab data and field data are not the same thing
This is the single most common source of the argument between your developer and your agency.
- Lab data comes from a simulated load, usually PageSpeed Insights or Lighthouse. It runs once, on a throttled connection, from a data centre. It is useful for debugging and it is not what Google uses to rank.
- Field data comes from the Chrome User Experience Report, collected from real Chrome users over 28 days. This is what feeds the ranking signal.
- A site can score 40 in the lab and pass in the field, or the reverse, and both readings are correct.
If you want the number that matters, look at the Core Web Vitals report in Search Console, not a Lighthouse score screenshot.
What actually moves each number
Fixing LCP
In our experience the cause is almost always one of four things: an oversized hero image, a render-blocking stylesheet or font, slow server response, or a carousel that loads five images when it needs one. Compress the hero, serve it in a modern format, preload it, and load fonts without blocking. That is most of the work.
Fixing INP
INP replaced First Input Delay in 2024 and it is stricter. The usual cause is too much JavaScript running on the main thread: heavy third-party tags, chat widgets, or an analytics stack with six vendors in it. Audit what you are loading and remove what nobody reads.
Fixing CLS
Set explicit width and height on images and video, reserve space for ads and embeds, and avoid injecting banners above existing content. Fonts that swap late also cause it, which is why font loading strategy matters more than it sounds.
How much do they actually affect rankings?
Less than most agencies imply when they are selling you a speed project. Google has been clear that page experience is a tiebreaker rather than a primary signal: better content on a slower page usually still wins.
The commercial case is stronger than the ranking case. Slow pages lose visitors before they see anything, and that shows up in conversion rate long before it shows up in position. We treat speed as a conversion problem that happens to have a small SEO benefit.
What to ask your developer
- Are we passing in Search Console field data, not just Lighthouse?
- Which metric is failing, and on mobile or desktop?
- What is the single largest contributor to LCP on our slowest template?
- How many third-party scripts load on every page, and who owns each one?
- What performance budget are we holding to on new work?
A developer who can answer these has measured. One who sends a screenshot of a green score has not.
Frequently asked questions
What is a good Core Web Vitals score?
LCP under 2.5 seconds, INP under 200 milliseconds, CLS under 0.1, achieved for at least 75 percent of real visits. All three have to pass. Anything between those thresholds and roughly double them is a needs-improvement result, and beyond that is a fail.
Do Core Web Vitals affect SEO?
Yes, but modestly. They are part of page experience, which Google treats as a tiebreaker between pages of similar relevance rather than a primary ranking factor. Content quality and links matter far more. The stronger argument for fixing them is conversion, not position.
Why does PageSpeed Insights disagree with Search Console?
PageSpeed runs a single simulated test from a data centre. Search Console reports real Chrome users over 28 days. They measure different things and both can be right at once. Search Console is the one tied to ranking.
Is WordPress bad for Core Web Vitals?
No. Badly built WordPress is. Plugin weight, page builders and cheap hosting are the usual causes, and none of them are inherent to the platform. A custom theme on decent hosting passes comfortably.
How long do improvements take to show?
Field data is a rolling 28-day window, so expect roughly four weeks after a fix before Search Console reflects it fully. Lab tools show the change immediately, which is why teams often think a fix failed when it simply has not aggregated yet.