About Core Web Vitals
Understanding Google's key performance metrics
Looking for more tools? Check out Link Analyzer for SEO link analysis.
What are Core Web Vitals?
Core Web Vitals are a set of standardized metrics from Google that help developers understand how users experience a web page. These metrics focus on three key aspects of user experience: loading performance, interactivity, and visual stability.
Since 2021, Core Web Vitals have been part of Google's page experience signals used in search ranking. This means sites with better Core Web Vitals scores may receive a ranking boost in search results.
Largest Contentful Paint (LCP)
LCP measures loading performance. It marks the point when the largest content element (like an image or text block) becomes visible in the viewport.
First Input Delay (FID)
FID measures interactivity. It quantifies the experience users feel when they try to interact with the page - specifically, the delay between when a user first interacts and when the browser responds.
Cumulative Layout Shift (CLS)
CLS measures visual stability. It quantifies how much unexpected layout shift occurs during the page's entire lifespan. Ever clicked a button and then the page shifted, causing you to click something else? That's a layout shift.
Why Core Web Vitals Matter
SEO Impact: Sites with better Core Web Vitals may rank higher in Google search results
User Experience: Better metrics mean faster, more responsive, and more stable pages for users
Conversion Rates: Studies show faster sites have higher conversion rates and lower bounce rates
Competitive Advantage: Most sites still have poor Core Web Vitals - optimizing yours gives you an edge
Frequently Asked Questions
How often should I test my Core Web Vitals?
We recommend testing weekly during active development and monthly for stable sites. Core Web Vitals can change due to content updates, design changes, or third-party script modifications. Regular testing helps you catch performance regressions early.
What's the difference between lab data and field data?
Lab data comes from simulated tests in controlled environments (like PageSpeed Insights). Field data comes from real Chrome users visiting your site (Chrome UX Report). Lab data is useful for debugging, while field data reflects actual user experience. Google uses field data for search rankings.
Why do my scores differ from Google PageSpeed Insights?
Test results can vary based on testing location, network speed, server load, and caching. Both tools use the same APIs, but timing differences and network conditions cause score variations. Run multiple tests and look for trends rather than focusing on single scores.
Does CoreVitals.io store my test results?
All test results are stored locally in your browser using IndexedDB. We never send your data to our servers. Your history is private and only accessible from your browser. Clear your browser data to remove all test history.
What should I do if my Core Web Vitals are "Poor"?
Start by testing your site and reviewing the optimization recommendations. Common fixes include: optimizing images (compress, use WebP, add lazy loading), reducing JavaScript execution time, setting width/height attributes on images to prevent layout shifts, and using a CDN for faster content delivery.
Is there a limit to how many tests I can run?
CoreVitals.io uses Google's PageSpeed Insights API, which has generous rate limits (25,000 requests per day). If you encounter a "Too many requests" error, wait 1-2 minutes before testing again. For high-volume testing, consider using your own API key.
Can I test localhost or private URLs?
No, the PageSpeed Insights API can only test publicly accessible URLs. For local development testing, use browser DevTools' Lighthouse panel (Chrome DevTools > Lighthouse tab) or install the web-vitals library to measure metrics during development.
How long does Google's Chrome UX Report data take to update?
CrUX data represents a rolling 28-day average of real user metrics. Changes to your site won't immediately reflect in field data - it can take several weeks for improvements to show. Lab data updates immediately, making it better for testing changes.
Should I optimize for mobile or desktop first?
Focus on mobile first. Google uses mobile-first indexing, meaning mobile performance has more SEO impact. Most web traffic comes from mobile devices, and mobile users are more sensitive to slow performance. Test both, but prioritize mobile optimization.
What's the difference between FID and INP?
FID (First Input Delay) measures only the first interaction on a page. INP (Interaction to Next Paint) measures all interactions throughout the page's lifetime and is replacing FID as a Core Web Vital in 2024. Both measure responsiveness, but INP gives a more complete picture.
Disclaimer
CoreVitals.io is a free tool that provides performance testing and optimization recommendations for informational and educational purposes only. Test results may vary based on network conditions, server load, geographic location, and testing methodology. This tool should not be used as the sole basis for critical business, SEO, or development decisions. Always verify findings with multiple testing tools and consult with qualified web performance professionals before making significant changes to your website. We do not guarantee the accuracy, completeness, or timeliness of any information provided by this tool.