Core Web Vitals: What They Are and Why They Affect Your Google Ranking

Core Web Vitals are three metrics that Google has used as ranking factors since May 2021. They aren’t the only factors, but they are the most technical and the most directly controllable by a developer. A website with poor Core Web Vitals scores can lose positions in Google results to competitors with similar content but a better technical experience.

If you have a WordPress website and you haven’t heard of LCP, INP or CLS, or you’ve heard of them but don’t know what to do about them, this article explains what matters in practice.

What Core Web Vitals measure

Google chose three aspects of the user experience to measure in a standardized way: how quickly the main content appears, how fast the page responds to interactions, and how stable the layout is while the page loads.

Each aspect has a specific metric with clear thresholds: below a certain number you’re in the green zone, between two values you’re in the yellow zone (needs improvement), and above the upper threshold you’re in the red zone (poor).

The data isn’t simulated. Google collects real data from Chrome users through the Chrome User Experience Report (CrUX) and uses it in the ranking algorithm. That means performance on your users’ real devices matters, not just the score from a lab test.

LCP: Largest Contentful Paint

LCP measures how long it takes for the largest visible element in the viewport to load completely. In practice, it’s usually the hero image on the homepage, a large product photo or the main text block of an article.

The thresholds: under 2.5 seconds is good, between 2.5 and 4 seconds needs improvement, over 4 seconds is poor.

The most common causes of a poor LCP are a large, unoptimized hero image, a missing preload for the LCP image, and a high TTFB (Time to First Byte) caused by slow hosting.

How to improve LCP: compress the hero image and convert it to WebP, add fetchpriority="high" to the LCP image, make sure the image isn’t lazy-loaded (images above the fold must not use lazy loading), and improve TTFB through server caching or a hosting upgrade.

INP: Interaction to Next Paint

INP replaced FID (First Input Delay) in March 2024. It measures how quickly the page responds to all user interactions: clicks, taps and key presses. It takes into account all interactions in the session, not just the first one.

The thresholds: under 200ms is good, between 200 and 500ms needs improvement, over 500ms is poor.

A poor INP usually comes from heavy JavaScript that blocks the browser’s main thread. On WordPress, the main culprits are plugins with poorly optimized JS code, large JavaScript libraries and tracking code (Google Analytics, Facebook Pixel, heatmaps).

How to improve INP: remove plugins with unnecessary JS, defer third-party scripts (tracking, chat, external widgets), and use Google Tag Manager to load analytics scripts asynchronously.

CLS: Cumulative Layout Shift

CLS measures how much elements move around on the page while it loads. A button that jumps when an ad appears above it, or text that shifts when the web font loads, are classic examples of layout shift.

The thresholds: under 0.1 is good, between 0.1 and 0.25 needs improvement, over 0.25 is poor. A value of 0 means nothing moves.

The most common causes: images without width and height attributes defined in the HTML (the browser doesn’t know how much space to reserve before loading), web fonts that replace the fallback font and change the size of the text, and ads or embeds without fixed dimensions.

How to improve CLS: always add width and height to img tags, use font-display: swap for web fonts and specify a fallback font with similar dimensions, and reserve fixed space for any asynchronously loaded element (ads, embeds).

How to measure Core Web Vitals for your website

There are several tools, each with a different perspective.

PageSpeed Insights — the most accessible. Enter the URL and you get separate scores for mobile and desktop, with specific recommendations. It combines lab data with real CrUX data if the website has enough traffic.

Google Search Console — in the “Core Web Vitals” section you’ll find aggregated data for all the pages on your website, classified as Good, Needs Improvement and Poor. It’s the best place to identify problem pages at scale.

Chrome DevTools Lighthouse — a detailed in-browser audit, useful for specific debugging. Press F12 in Chrome, open the Lighthouse tab and run an audit on Mobile.

Prioritize improvements on mobile. Google indexes and ranks primarily based on the mobile version of your website, and mobile scores are almost always worse than desktop ones.

Core Web Vitals on WordPress: practical priorities

On a typical WordPress website, the following order of priorities solves most Core Web Vitals problems.

First, optimize your images: compress everything on your website, convert to WebP, and add width and height to all img tags. This step alone can dramatically improve both LCP and CLS.

Second, install a caching plugin with CSS/JS optimization. WP Rocket or LiteSpeed Cache can defer non-critical JavaScript, combine files and enable preloading for key pages.

Third, audit your active plugins. Deactivate what you don’t use, replace plugins with heavy JS with lighter alternatives and move third-party scripts into Google Tag Manager.

Fourth, check your web fonts. Preload the critical fonts and set font-display: swap to eliminate FOIT (Flash of Invisible Text).

Frequently asked questions

Do Core Web Vitals really matter for SEO, or is it exaggerated?

Core Web Vitals are ranking factors confirmed by Google, but they aren’t the only ones and probably not the most important. Relevant content and domain authority weigh more. But when two websites are comparable in content and authority, Core Web Vitals can make the difference. And regardless of SEO, a fast website retains visitors better.

I have a good score on desktop but a poor one on mobile. What should I do?

Mobile is more important for SEO (Google’s mobile-first indexing). Mobile-specific problems are usually large images on small screens, JavaScript running on weaker hardware and a layout that isn’t optimized for touch. Test on a real phone, not just in an emulator, and prioritize mobile optimizations.

Does a PageSpeed score of 100 mean my website is perfect?

Not necessarily. The lab PageSpeed score measures simulated performance under controlled conditions. Real Core Web Vitals, collected from real users on different devices and connections, can be different. A PageSpeed score of 85–90 with good real-world data in Search Console is worth more than a simulated score of 100.

How often should I check Core Web Vitals?

Check after every major change on your website: theme update, new plugin, new advertising campaign, redesign. Normally, a monthly check in Search Console is enough to catch problems before they affect SEO.

Can I improve Core Web Vitals without a developer, just with plugins?

Yes, caching and optimization plugins (WP Rocket, LiteSpeed Cache) solve many problems automatically. Image optimization with ShortPixel or Imagify is just as accessible. More complex CLS or INP problems caused by custom code or a page builder usually require a developer.

Conclusion

Core Web Vitals aren’t technical complexity reserved for developers. They are three numbers that measure what the user feels when using your website, and Google uses them to decide whether you deserve a good position in the results.

LCP under 2.5s, INP under 200ms and CLS under 0.1 are the targets. On a typical WordPress website, image optimization and a good caching plugin solve 80% of the problems without touching the code.

If your scores stay poor after the basic optimizations, the problem is probably in the theme or the technical structure of the website. The Nexium Web team can run a complete technical audit and tell you exactly what needs to change. Contact us or learn more about our technical WordPress services.