Core Web Vitals

The Three Metrics and Their Thresholds

Core Web Vitals consist of three separate metrics, each with a good and a poor threshold. A page passes a metric when it falls in the good range for at least 75% of real user visits. You cannot average the three into one score.

LCP tracks the time until the largest visible element (image, text block) renders. An LCP over 4.0 seconds is poor. INP measures the delay between a user interaction (click, tap, keypress) and the next paint. An INP above 500 milliseconds is poor. CLS quantifies unexpected layout shifts; a score above 0.25 is poor.

• LCP: good ≤ 2.5s, poor > 4.0s
• INP: good ≤ 200ms, poor > 500ms
• CLS: good ≤ 0.1, poor > 0.25

Lab Data vs Field Data

Lab data comes from a controlled test using a simulated device and network. Tools like Lighthouse produce lab scores. Field data, or Real User Monitoring (RUM), collects metrics from actual visitors. The Core Web Vitals assessment uses field data exclusively.

Lab data can help you diagnose issues, but it may not reflect real-world conditions. Always check the Chrome User Experience Report (CrUX) in Google Search Console or PageSpeed Insights for field data. A page can pass lab tests yet fail in the field if users have slow connections or older devices.

For example, a lab test on a fast Wi-Fi connection might show an LCP of 1.5 seconds, but field data from users on 3G networks could show an LCP of 3.5 seconds, failing the threshold. Similarly, a site optimised for desktop lab tests might have high CLS on mobile devices due to unresponsive images. Understanding this gap is critical for accurate optimisation. Another common scenario is that a JavaScript-heavy site performs well in lab tests with a powerful machine but causes poor INP on low-end phones in the field.

Impact on Search Rankings

Google includes Core Web Vitals as one of several page experience signals. A page that performs well on these metrics may receive a small ranking boost, especially in competitive scenarios where other factors are equal. However, good Core Web Vitals alone cannot compensate for thin content or poor relevance.

The importance of these signals has been debated. Google states that page experience is a ranking factor, but it remains secondary to the overall relevance and quality of content. For SEO practitioners, the practical value is often in diagnosing user experience issues that hurt conversion rates, not just rankings.

For instance, a site with excellent Core Web Vitals but low-quality content will not outrank a site with slightly worse metrics but authoritative, relevant content. This aligns with how how google algorithm update works — each update refines the balance between user experience signals and content quality. Additionally, understanding how seo performance works helps contextualise Core Web Vitals within broader ranking factors.

Common Optimisation Mistakes

Many site owners focus on a single metric while ignoring the others. This frequently leads to missed opportunities or even regressions in other areas. For example, adding a lazy-load script to improve LCP might increase CLS if dimensions are not reserved. Another mistake is relying on outdated documentation. Because FID was replaced by INP in March 2024, any audit or tool that still references FID is likely using obsolete thresholds. Always verify current guidance from Google Search Central.

Additionally, consider that Page Speed optimisation involves more than just these three metrics.

A further error is neglecting the impact of third-party scripts. Embedding a heavy analytics tool or social media widget can degrade INP significantly, yet many developers overlook this. For instance, a site that loads a large chat widget on every page might see INP rise from 150ms to 400ms, pushing it into the poor range. Another common oversight is not testing on real mobile devices. A developer might optimise for a desktop browser but miss that a mobile user's interaction with a carousel causes layout shifts. Using a seo cdn can help by serving static assets faster, but it must be configured correctly to avoid introducing latency. Finally, some teams focus only on the homepage, ignoring that product or article pages may have worse metrics due to heavy images or dynamic content.

• Treating Core Web Vitals as a single score: fails to identify which metric needs work.
• Using FID in current documentation: uses a deprecated metric that no longer applies.
• Relying only on lab tools: misses real-world field data that determines pass/fail.
• Assuming good CWV compensates for weak content: ignores Google's emphasis on relevance.
• Ignoring third-party scripts: can silently degrade INP without obvious cause.
• Not testing on real mobile devices: misses CLS issues from responsive layouts.

Tools to Monitor Core Web Vitals

Google Search Console provides a dedicated Core Web Vitals report that shows which URLs are passing or failing each metric. The report uses field data from CrUX. Google Search Console is the most direct way to see how Google views your site's user experience.

PageSpeed Insights reports both lab and field data for a single URL. It also shows the specific elements that contribute to LCP, CLS, or INP. For ongoing monitoring, consider using the CrUX API or a real-user monitoring service. monitoring seo performance tools can help you track changes over time.

Additionally, tools like Lighthouse in Chrome DevTools allow you to audit a page locally, but remember that this is lab data. For a comprehensive view, combine lab tests with field data from CrUX. For example, you might use Lighthouse to identify that an image is the LCP element, then check CrUX to see if real users experience slow LCP on that page. Another useful approach is to set up alerts in Google Search Console for when a metric drops below the good threshold, allowing proactive fixes. For sites built with frameworks like Next.js SEO, monitoring server-side rendering performance is crucial, as poor TTFB can inflate LCP. Understanding SEO Metrics as a whole helps you prioritise which Core Web Vitals issues to address first based on their impact on user experience and rankings.