What Are Core Web Vitals? The Complete 2025 Guide

Why Core Web Vitals matter for SEO

Google officially incorporated Core Web Vitals into its ranking algorithm in 2021 as part of the Page Experience update. In 2024, Interaction to Next Paint (INP) replaced First Input Delay (FID) as the third metric. Together, these three signals measure how real users experience your pages — not just how fast they technically load.

A poor Core Web Vitals score doesn't just hurt ranking — it hurts conversion. Google's own research shows that pages meeting Core Web Vitals thresholds see a 24% lower abandonment rate than those that don't.

The three Core Web Vitals explained

Largest Contentful Paint (LCP)

LCP measures how long it takes for the largest visible content element — usually a hero image or heading — to render on screen. Google's threshold:

• Good: under 2.5 seconds
• Needs improvement: 2.5 – 4.0 seconds
• Poor: over 4.0 seconds

The most common LCP killers are render-blocking resources, slow server response times (TTFB), and unoptimised images without loading="eager" and explicit width/height attributes on the above-the-fold image.

Interaction to Next Paint (INP)

INP replaced First Input Delay in March 2024. Where FID only measured the delay before the browser started processing an interaction, INP measures the full duration from user interaction to the next visual update. Good INP is under 200 ms. Over 500 ms is poor. Heavy JavaScript on the main thread — especially third-party scripts — is the leading cause of poor INP.

Cumulative Layout Shift (CLS)

CLS quantifies how much the page layout shifts unexpectedly during load. A score of 0.1 or lower is good; above 0.25 is poor. The most common cause is images or embeds without declared dimensions, and web fonts that cause text to reflow once they load (FOUT).

How to measure Core Web Vitals

There are two types of data: lab data (simulated, instant) and field data (real users, collected over 28 days). Google uses field data for ranking — specifically the 75th percentile of all page loads on your site.

• PageSpeed Insights — shows both lab and field data for any URL
• Google Search Console — Core Web Vitals report across your whole site
• Chrome DevTools — Performance panel for lab debugging
• ScanoraAI — runs a live PSI audit and surfaces the exact elements causing each metric to fail

Quick wins to improve each metric

LCP: Preload the hero image (<link rel="preload" as="image">), use a CDN, and compress images to WebP. Aim for TTFB under 800 ms.

INP: Defer non-critical JavaScript, move heavy work off the main thread with Web Workers, and audit third-party tag payloads.

CLS: Add explicit width and height to every image and embed, use font-display: optional or preload fonts, and avoid inserting DOM nodes above existing content.

The bottom line

Core Web Vitals are measurable, fixable, and directly tied to both ranking and revenue. Run a free audit on ScanoraAI to see exactly where your site stands and get a prioritised fix list in under 60 seconds.