Core Web Vitals: You think your site is fast? Google may not agree

You track your pages' performance, you've heard of Core Web Vitals, but with all the available tools it's hard to know where to click, what to look at, or when. Don't panic, we'll sort it out together.

What are Core Web Vitals?

For years, web performance was measured with indicators like TTFB or the number of JavaScript requests. But with the arrival of HTTP/2, the rise of front-end frameworks and the evolution of browsers, these metrics have lost relevance. It's no longer just a question of raw speed, but of user perception.

That's why Google introduced the Core Web Vitals : three metrics that concretely reflect what a user perceives when browsing a page. Does it load quickly? Is it stable? Does it respond well when clicked?

Here are the three main metrics:

• INP (Interaction to Next Paint) : since March 2024, INP replaced FID. It measures the delay between a user interaction (click, tap) and the moment the site shows a visual response. It is a strong signal of responsiveness.

• LCP (Largest Contentful Paint) : it measures when the largest visible element on the page (often an image or a heading) fully appears in the viewport. A good LCP often depends on the template, the device, or even a detail like a consent pop-up.

• CLS (Cumulative Layout Shift) : this metric assesses visual stability. It detects unexpected shifts throughout the page's lifetime, not just at load. Typically, a button that moves just as you were about to click it.

CrUX: real numbers from real users

What a performance tool shows you locally doesn't necessarily reflect your visitors' experience. Those are lab tests, in a standardized environment. But Google looks at something else: real-world, field data. And that's where CrUX comes into play.

CrUX, to Chrome User Experience Reportit is a public database fed by Chrome browser users. It collects performance metrics real, in everyday browsing conditions: a typical smartphone, 4G that isn’t always stable, background tabs open, etc.

These data are referred to as "field", as opposed to data "lab", generated locally with tools like Lighthouse. CrUX data are what Google uses to judge your Core Web Vitals in its algorithm. In other words, a good PageSpeed score guarantees nothing if the CrUX data are poor. It is the latter that truly matter.

You can view this data:

• Via PageSpeed Insights, if your URL gets enough traffic
• In Search Console, with the dedicated "Core Web Vitals" report
• Or more advanced via theCrUX API or BigQuery, if you need to run large-scale analyses

Note that CrUX does not cover all pagesA certain volume of traffic is required for a URL to appear in the database. For recent, little-visited, or very niche sites, some pages simply won’t be measured. But at the scale of an established site, it’s an excellent reflection of the real user experience.

11 tools to measure and track your Core Web Vitals

There are many tools to measure Core Web Vitals, but they don’t all serve the same purpose. Some are useful for one-off audits, others for continuous monitoring, and a few are designed for deeper technical integration. This table helps you see things more clearly, according to your need and skill level.

4 tools recommended by a web performance expert

Chrome DevTools

Essential for visualizing what the user actually sees and understanding why why a page is performing poorly. Perfect for analyzing an LCP or a CLS on the fly.

Chrome DevTools is an ideal tool for debugging Core Web Vitals in lab mode thanks to the “Performance” tab.
It allows you to analyze the potential data that might be sent to BigQuery to feed the Chrome UX Report. In addition to precisely identifying the LCP element, it is also one of the best tools for diagnosing measured KPIs live throughout the page lifecycle, such as CLS and INP.

Aymeric Bouillat

Treo.sh

Ideal for tracking the evolution of real Core Web Vitals over time, with a clear interface that avoids getting lost in a technical dashboard.

Treo.sh offers the advantage of quickly identifying the templates most affected by poor Core Web Vitals scores, making it easier to prioritize actions for each KPI. Its real strength lies in its dynamic detection of URLs with data for a specific domain.

Aymeric Bouillat

DebugBear

Perfect for continuous monitoring, spotting regressions, and integrating performance into a product workflow or CI/CD.

DebugBear, which lets you track both CrUX data and lab data via the PageSpeed Insights API, is ideal for monitoring web performance KPI progress as technical improvements are made to a site. It also provides detailed recommendations to favor loading critical resources above the fold in the “Waterfall” view (cascading file loading).

Aymeric Bouillat

RUMvision (CWV competitor comparison)

Measure, analyze, and improve your site's speed and essential web metrics using real-time visitor data.

Knowing that your site feels fast and provides a good user experience is great, but knowing how you compare to the competition is even better. That’s what CWV Compare Competitors does: it shows how the site ranks against previously identified sites in the same sector. Ideal for web performance audits.

Aymeric Bouillat

Thanks to Aymerci Bouillat for their feedback and advice

Tools are good. Users are better.

Good indicators aren't there to stress you out, but to guide you. The real issue isn't getting 100/100 on Lighthouse, but understanding how your users experience your site every day on their own devices.

Core Web Vitals give you concrete benchmarks to identify what truly hinders the experience. There's no need to measure everything or follow ten dashboards: focus on the pages that matter, using tools you master. A good starting point is already a big part of the journey.

The article "Core Web Vitals: You think your site is fast? Google may not agree" was published on the site Abondance.