Core Web Vitals are three measurements Google takes of how your page feels to a real person: how long until the main content appears, how quickly the page responds when they interact, and how much the layout jumps around while loading. They are a ranking factor, a modest one, and they matter considerably more for conversion than for rankings.
Here is what each one means, and an honest ranking of which are worth spending money to fix.
Largest Contentful Paint
What it measures. How long until the biggest visible thing on screen finishes loading. Usually a hero image or a headline.
The threshold. Under 2.5 seconds is good. Over 4 seconds is poor.
In plain terms. How long the visitor stares at something that is not yet your website.
Usual causes. Oversized images, slow server response, render blocking CSS and JavaScript loading before anything can display, and fonts that delay text appearing.
Worth fixing? Yes, first, always. This is the one that correlates most directly with people leaving, and it is usually the cheapest to improve. Correctly sizing and converting images alone fixes a large share of failing websites.
Interaction to Next Paint
What it measures. When someone taps or clicks, how long before the page visibly responds.
The threshold. Under 200 milliseconds is good. Over 500 is poor.
In plain terms. Whether your site feels broken when someone touches it. The specific experience is tapping a menu and wondering whether it registered.
Usual causes. Too much JavaScript competing for the main thread. Almost always a plugin or a page builder rather than anything you wrote.
Worth fixing? Yes, if you are failing it, though the fix is often structural. On a heavily plugged WordPress site this frequently cannot be resolved without removing the plugins responsible.
Cumulative Layout Shift
What it measures. How much content moves around while the page loads.
The threshold. Under 0.1 is good. Over 0.25 is poor.
In plain terms. Whether the button moves just as someone goes to press it. Everyone has experienced this and everyone finds it infuriating.
Usual causes. Images and embeds without width and height set, so the browser does not reserve space. Ads and banners injected above existing content. Web fonts that swap and change the height of text. Sliders, reliably.
Worth fixing? Yes, and it is usually the cheapest of the three. Setting dimensions on images fixes most of it and takes an afternoon, and it is felt most on phones, where a shift moves the entire screen.
The honest ranking
- Largest Contentful Paint. Highest impact on whether people stay, usually the cheapest meaningful win.
- Cumulative Layout Shift. Cheap to fix, noticeably improves how the site feels.
- Interaction to Next Paint. Matters, but the fix is often “use fewer plugins”, which is a project rather than a task.
The trap worth avoiding
There are two kinds of measurement. Lab data comes from a simulated load, which is what a testing tool shows you on demand. Field data comes from real visits by real people on real devices and connections, and it is what Google actually uses.
They frequently disagree, and the disagreement always runs the same direction: lab data looks better. A site can score well in a test and fail in the field, because your visitors are on worse phones and worse connections than the test assumes.
This is also how a caching plugin can improve your score without improving anyone’s experience. Trust the field data. It is in Search Console under Core Web Vitals and it is the number Google is using.
Questions
How much do these actually affect rankings?
Less than most people selling speed optimization imply. They are a tiebreaker between pages of comparable relevance, not a way to outrank better content. The conversion argument is much stronger than the ranking argument, and it is the one worth making.
My score is 100 in one tool and 60 in another. Which is right?
Neither, exactly. Both are lab simulations with different assumptions. Field data in Search Console is the one that counts.
Can I fix this without rebuilding?
Often, yes. Image handling, dimensions, and removing unnecessary scripts get most sites into passing range. A rebuild becomes the answer when the cause is the page builder itself, because that is not something you can configure your way out of.
This site publishes its own measurements on the Under the Hood page, and if you want yours measured properly rather than guessed at, that is where a technical engagement starts.
