How to Monitor Core Web Vitals: FCP, LCP, and CLS Explained

What are Core Web Vitals?

Core Web Vitals are a set of performance metrics defined by Google that measure the user experience of loading a web page. Google uses them as ranking signals, which means poor Core Web Vitals can hurt search visibility in addition to degrading the user experience.

The three primary Core Web Vitals are First Contentful Paint (FCP), Largest Contentful Paint (LCP), and Cumulative Layout Shift (CLS). Each measures a different dimension of how a page feels to load and use.

Why Core Web Vitals affect more than SEO

Google has incorporated Core Web Vitals into its page experience ranking signal, so pages with poor performance can rank lower than pages with equivalent content but faster load times. That makes Core Web Vitals a direct SEO concern for any page targeting organic traffic.

Beyond rankings, slow FCP and LCP directly increase bounce rates. Research consistently shows that pages taking over three seconds to load lose a significant share of mobile visitors before the content even renders. CLS failures cause visitors to click the wrong element when layout shifts mid-interaction — a checkout button that jumps can trigger accidental navigations.

For SaaS products and ecommerce stores, a Core Web Vitals regression after a deploy can reduce conversions before anyone notices, because the page is technically still online and passing uptime checks.

How NorthDuty monitors Core Web Vitals

Every NorthDuty health check run captures FCP, LCP, and CLS alongside the full performance timing breakdown: DNS lookup, TCP connection, TLS handshake, time to first byte, content download, DOM parsing, DOMContentLoaded, and full load time.

These metrics are collected using Chromium, the same browser engine Google uses for its own measurement, so the values reflect what real visitors experience rather than server-only response times.

When performance metrics degrade — for example, if LCP rises above 2.5 seconds after a deploy — NorthDuty captures the change as a health finding with a performance category and the appropriate severity level. Teams can configure a response_time_above alert rule to get notified when response time crosses a custom threshold.

Best practices for maintaining good Core Web Vitals

Set a performance budget for LCP on your highest-traffic pages — typically under 2.5 seconds — and monitor it after every deploy. NorthDuty checks every 1 minute by default, so regressions surface quickly.

Audit your page for images without explicit dimensions before launch. That single fix eliminates the most common source of CLS regressions.

Pair Core Web Vitals monitoring with user journey monitoring. A page can have good performance metrics while still failing at step three of a checkout flow. Both layers are needed for a complete picture of the customer experience.