Core Web Vitals

What it is

Largest Contentful Paint should be 2.5 seconds or less, Interaction to Next Paint 200 milliseconds or less, and Cumulative Layout Shift 0.1 or less, measured at the 75th percentile of real visits. INP replaced First Input Delay as a Core Web Vital in March 2024. Field data comes from CrUX; lab tools like Lighthouse help find causes.

Why it matters

They are part of Google’s page experience signals and, more importantly, they shape whether visitors stay and convert. Slow or unstable pages cost users before they cost rankings.

Common mistakes

• Optimizing lab scores while real-user (field) data stays poor
• Oversized hero images and render-blocking scripts hurting LCP
• Images and embeds without reserved space causing layout shifts
• Heavy third-party scripts blocking interaction (INP)

How I handle it

I start from CrUX and RUM data to see what real users experience, find the real cause with lab tests, and hand your developers a brief naming the exact templates and rules to change.

How to check it

1. Open the Core Web Vitals report in Search Console and PageSpeed Insights to see real-user (CrUX) data.
2. Run Lighthouse to find the causes: the LCP element, layout shifts and long tasks.
3. Test on a mid-range phone and a slow connection, not only on a fast laptop.
4. Set up real-user monitoring if you need results by page template or country.

Illustrative example

Example scenario: the largest element on a category page is a 2 MB hero image loaded late, so LCP is 4 seconds on mobile. Serving a properly sized modern image and preloading it brings LCP down without touching the design.

Frequently asked questions