How to Optimize for Core Web Vitals: A Complete Guide

Search engine optimization (SEO) is constantly evolving to provide users with the best possible online experiences. In 2020, Google introduced a new set of metrics called Core Web Vitals that focus on measuring real-world user experiences on websites.

Optimizing for Core Web Vitals should now be a top priority for anyone doing SEO and web development. High Core Web Vitals scores lead to better user experiences, which Google rewards in search rankings.

This comprehensive guide will teach you everything you need to know about Core Web Vitals and how to optimize your site for high scores.

Key Takeaways:

• Core Web Vitals measure real-world user experiences on 3 key dimensions: loading, interactivity, and visual stability.
• Google has confirmed Core Web Vitals are a ranking factor. High scores lead to better rankings.
• Tools like PageSpeed Insights, Lighthouse, and Chrome User Experience Report can measure your Core Vitals scores.
• Improve Core Web Vitals by optimizing pages for speed, minimizing React, implementing PRPL pattern, using Core Web Vitals as metrics inperformance budgets, and more.
• Core Web Vitals scores vary based on device and network conditions. Optimize for the lower end of capable devices.
• Core Web Vitals align well with existing best practices like performance optimization and responsive design.

What Are Core Web Vitals?

Core Web Vitals are a set of metrics that measure real-world user experiences on websites. They focus on three aspects of experience:

Loading Experience

• Largest Contentful Paint (LCP) – measures loading performance

Interactivity

• First Input Delay (FID) – measures interactivity

Visual Stability

• Cumulative Layout Shift (CLS) – measures visual stability

Google introduced Core Web Vitals because they found a connection between higher scores and better business outcomes.

For example, sites with faster LCP scores tended to have higher conversion rates and lower bounce rates. Users are more likely to stay on sites that load quickly.

Core Web Vitals are now a Google ranking factor. Having high Core Web Vitals scores will lead to better search rankings and visibility.

The scores help developers quantify user experiences so they can improve them. Evaluating Core Web Vitals shows you exactly how real users experience your site.

Why Core Web Vitals Are Important for SEO

Google has explicitly stated that Core Web Vitals are used as a ranking signal.

In May 2020, Google announced that the page experience signals, including Core Web Vitals, would become a ranking factor in 2021.

In March 2021, Google confirmed that page experience ranking factors are now in use.

High Core Web Vitals scores positively influence search rankings and visibility. Sites with poor user experiences will be less likely to rank well.

Even a small increase in Core Web Vitals scores can improve search rankings. Page experience signals have a 12% overall search ranking factor weight according to Moz.

Optimizing for Core Web Vitals aligns with Google’s overall mission – to provide the most relevant and usable content in search results.

Users have come to expect fast, interactive, and visually stable sites. Optimizing Core Web Vitals improves real-world user experiences, which Google recognizes with better rankings.

How Google Measures Core Web Vitals

Google uses real-world user data from multiple sources to measure Core Web Vitals scores for sites.

The primary data source is Chrome User Experience Report (CrUX). This aggregates anonymous Chrome telemetry data from millions of actual site visitors.

Google also uses data from Lighthouse to supplement CrUX data. Lighthouse is an automated tool that simulates user experiences.

Lab data from WebPageTest can also provide additional data on Core Web Vitals measurements.

Based on these data sources, Google calculates percentile scores for each Core Web Vitals metric:

• Good – 75th percentile and above
• Needs Improvement – 50th – 75th percentile
• Poor – Below 50th percentile

Google rewards sites in the “Good” range, especially those in the 90th percentile and above.

The targets will vary based on device and network conditions. Faster devices on fast networks will have higher targets.

How to Measure Core Web Vitals

There are several tools you can use to measure your site’s Core Web Vitals scores:

PageSpeed Insights

The easiest way to get started is PageSpeed Insights.

Enter a URL and PageSpeed Insights will analyze that page and show your Core Web Vitals scores.

It also provides suggestions to improve each one.

Lighthouse

Lighthouse is an automated tool that runs audits for performance, accessibility, SEO and more.

You can run Lighthouse reports locally or through third-party tools like WebPageTest and browser extensions.

Lighthouse shows your Core Web Vitals scores in the report.

Chrome User Experience Report

The Chrome User Experience Report (CrUX) shows how real users experience your site on Chrome.

Connect your Google Analytics and Google Search Console accounts to see your CrUX data in those tools.

You can view your percentile scores compared to other sites along with the distributions of Core Web Vitals scores from actual users.

Sitespeed.io

Sitespeed.io is an open-source tool that measures performance using WebPageTest and Browsers like Chrome and Firefox.

It generates HTML reports that include your Core Web Vitals scores.

Search Console

In Google Search Console, you can view your Core Web Vitals scores under the Enhancements report.

Connect Search Console to Google Analytics to segment users by location, device type, and more.

Chrome DevTools

Chrome DevTools has several panels that help measure Core Web Vitals:

• Performance – View LCP and FID scores
• Experience – Simulate slow connections to view metrics
• Rendering – View layout shifts

Real User Monitoring

Real user monitoring tools like Calibre, SpeedCurve and Akamai mPulse give you Core Web Vitals data from real visitors after deployment.

Synthetic monitoring tools can also provide additional lab and testing data.

How to Improve Core Web Vitals Scores

Optimizing for Core Web Vitals aligns closely with existing best practices like site speed optimization, caching, and responsive design.

Here are some tips to improve each of the Core Web Vitals metrics:

Largest Contentful Paint (LCP)

LCP measures how fast the main content of a page loads. To improve it:

• Minify HTML/CSS/JavaScript – Minification removes whitespace and unnecessary code to reduce file sizes.
• Compress images – Use tools like ImageOptim to optimize images.
• Lazy load images/iframes – Only load offscreen images when the user scrolls near them.
• Defer non-critical JS – Leverage defer and async attributes to delay non-critical JS.
• Preconnect to origins – Add preconnect or dns-prefetch to establish early connections to third-party origins.
• Cache static assets – Set far-future cache headers and leverage a CDN to cache assets near users.
• Preload key requests – Use <link rel="preload"> for critical assets to boost priority.
• Reduce server response times – Upgrade hosting plans, enable caching, or optimize code to speed up server response times.
• Load CRP faster – Use PRPL pattern or render-as-you-fetch to show content faster.
• Size content appropriately – Avoid overly large images, videos, fonts and other assets.

First Input Delay (FID)

FID measures how long it takes for a page to become interactive after a user first interacts with it. To improve it:

• Minimize JavaScript after interactions – Defer non-critical JS that may delay interactivity.
• Prerender with rehydration – Prerender HTML on the server and “rehydrate” it client-side to boost interactivity.
• Use passive event listeners – Add { passive: true } to touch and wheel event listeners.
• Reduce long tasks – Split up or defer any JS tasks longer than 50ms.
• Increase user input responsiveness – Ensure the main thread can process user input quickly by minimizing main thread work.
• Load critical fonts async – Load non-critical web fonts asynchronously.

Cumulative Layout Shift (CLS)

CLS measures the stability of content as it loads and shifts around. To improve it:

• Add dimensions to images/videos – Include width + height to prevent content shifts.
• Avoid large layout changes – Defer or sequence any changes to layout after the page is loaded.
• Use CSS Grid for layout – CSS Grid offers greater stability than floats.
• Reduce dynamically injected content – Avoid injecting content late in the page load to prevent shifts.
• Add layout shifts to animation budget – Account for any planned shifts in the page animation budget.
• Defer non-critical JS/CSS – Delay loading JS and CSS that triggers layout shifts when it loads.
• Avoid DOM mutations after load – Minimize changes to the DOM after the initial render, especially ones that trigger a reflow.

Advanced Tactics to Improve Core Web Vitals

In addition to general performance best practices, there are some advanced tactics specifically for improving Core Web Vitals scores:

Use PRPL Pattern

The PRPL pattern pushes rendering into edge networks for fast first delivery. For Core Web Vitals, it greatly improves LCP and FID scores.

1. Push critical resources for the initial route.
2. Render initial route ASAP.
3. Pre-cache remaining routes using service workers.
4. Lazy-load other routes on demand.

PRPL works well with most modern web frameworks.

Render-as-You-Fetch Pages

Render-as-you-fetch incrementally generates HTML and streams it into the browser. This allows users to see content faster.

Use a JavaScript framework like React Server Components that supports streaming HTML rendering.

Minimize React Hydration

With React, first render the app HTML on the server without data (no hydration). Then hydrate on the client after the HTML loads.

Avoid slowing down LCP by delaying hydration until after the first paint. Only hydrate components that need interactivity.

Use Core Vitals in Performance Budgets

Set targets for Core Vitals scores in your performance budgets. Alert if builds exceed those targets before deploying.

Aim for LCP under 1.5 seconds, FID under 100 milliseconds, and CLS under 0.1 on mobile networks according to WebPageTest research.

Regularly measure and monitor Core Vitals using RUM tools to detect regressions after deployments. Adjust targets as needed based on field data.

Optimize for Slower Networks/Devices

The targets for Core Web Vitals scores vary based on device capability and network conditions.

Make sure to test using:

• Budget Android devices like Moto G4
• Slow 3G network throttling (at least 700ms RTT and 1.6Mbps down)
• 4x CPU throttling

Optimize your site to meet targets for these slower scenarios. That will greatly improve scores for the majority of users.

How Core Web Vitals Align with UX Best Practices

Optimizing for Core Web Vitals isn’t just about chasing scores – it improves real user experiences.

Higher LCP directly improves load times, which reduces abandonment. Users are more likely to wait on pages with better loading performance.

Lower FID makes pages more responsive. Users can interact with the content faster after clicking links and buttons.

Minimizing CLS reduces unexpected shifts that disorient users as the page loads. A stable layout feels more polished.

On top of boosting conversions and engagement, optimizing Core Web Vitals has other UX benefits:

• Increased reach – A fast, stable site works better across slower devices and networks.
• Lower bounce rate – When pages load quickly with minimal layout shifts, visitors are less likely to leave.
• Improved SEO – Faster pages with responsive interactivity improve click-through-rates in search.
• Brand perception – A fast, polished loading experience reflects well on a brand.

Core Web Vitals and UX Research

Google has published extensive UX research that demonstrates how higher Core Web Vitals scores translate to happier users and better business results.

Some key findings:

• Pages with fast LCP scores have much higher conversion rates. A 100ms decrease in LCP correlated to a 1.67% increase in conversion rates across ecommerce sites analyzed.
• Poor FID scores negatively impact user engagement. A 100ms increase in FID led to a 0.62% decrease in user engagement across portals and news sites studied.
• High CLS scores increase bounce rates. For every 0.1 increase in CLS, ecommerce sites saw a 2.4% increase in bounce rate.
• Improvements add up quickly. Moving from the 25th to 75th percentile in all Core Web Vitals metrics combined correlated to an 11% increase in conversions and 8% decrease in bounce rates.

Research also shows how loading performance impacts user behavior:

• At 1 second delay, most users feel page loads are instantaneous.
• Delays of 3 seconds reduce satisfaction and engagement substantially.
• By 5 seconds, most users will abandon the page.
• By 10 seconds, almost all users have abandoned.

Optimizing all aspects of page experience, especially loading speed, is critical for maximizing conversions, engagement, and revenue.

FAQ

What is a good LCP score?

Google recommends LCP under 2.5 seconds generally. For ecommerce sites, aim for under 1.5 seconds for most users. The 75th percentile target varies based on device and network.

What is a good FID score?

Google recommends FID under 100ms. Sites should aim for under 50ms for most users, with a 75th percentile target around 100ms.

What is a good CLS score?

For CLS, aim for under 0.1 as the 75th percentile target. Most pages should try to maintain CLS under 0.25.

How much do Core Web Vitals factor into overall page experience and search rankings?

Google has not provided exact weights, but page experience ranking factors (including Core Web Vitals) are estimated to be around 12% of overall ranking signals according to Moz.

How often are Core Web Vitals scores updated?

Google reevaluates Core Web Vitals monthly but has not provided an exact cadence. Scores in tools like PageSpeed Insights and Search Console are updated on a rolling basis.

Can you manipulate Core Web Vitals scores while hurting real user experiences?

While possible with lab tricks, Google’s metrics are designed to reflect real user experiences. Actual field data from CrUX makes manipulation highly risky and unsustainable.

What is more important – lab scores or field data scores?

Optimizing for real user experiences based on field data is most important. Lab data helps identify issues to improve but reaching score targets in the lab alone is insufficient.

Conclusion

Core Web Vitals are vital metrics all sites must now optimize to provide the best possible user experiences.

Focus on improving LCP, FID and CLS by following web performance best practices like fast server response times, caching, and minimized JavaScript.

Measure Core Web Vitals frequently with tools like PageSpeed Insights. Monitor field data and set targets based on real user experiences.

Optimizing for Core Web Vitals has massive UX benefits while also boosting search rankings and visibility in competitive SERPs. Strive for at least “good” scores in search results to maximize clicks and conversions from organic traffic.

Following the latest performance optimization tactics is now mandatory. Use Core Web Vitals guidance, do thorough testing, set performance budgets, and align teams on driving continual improvements over time.

Shane Galvin

Shane Galvin is the founder of Blue Ocean Web Care, a WordPress maintenance and optimization company based in Rochester, NY. With 15+ years of experience in WordPress site security, speed optimization, and SEO, Shane utilizes his expertise to help clients build effective websites. His ultimate goal is to build fast, user-friendly websites that instill confidence and trust for clients.