Are Core Web Vitals (CWV) truly essential for SEO?

A bit of history

In May 2020, Google introduced the Core Web Vitals, defining them as:

“A set of metrics that measure real-world user experience in terms of loading performance, interactivity, and visual stability of the page.”

The goal was to quantify, as objectively as possible, key aspects that impact user experience. These were incorporated as ranking factors into Google’s search algorithm, emphasizing that a high-quality user experience is fundamental to good online visibility.

The 3 initial Core Web Vitals were:

• Largest Contentful Paint (LCP): measures loading performance. To provide a good user experience, LCP should occur within 2.5 seconds of when the page first starts loading.
• First Input Delay (FID): measures the time from a user’s first interaction with a page to the moment the browser is able to respond to that interaction (e.g., clicking a link or tapping a button).
• Cumulative Layout Shift (CLS): measures visual stability. To offer a good user experience, pages should maintain a CLS score of 0.1 or less.

Google has consistently emphasized the importance of Core Web Vitals through communications and algorithm updates, providing tools and reports (like Lighthouse, PageSpeed Insights, and the dedicated report in Google Search Console) to help site owners monitor and improve these metrics.

The most notable update came in March 2024, when Google replaced FID with INP:

• Interaction to Next Paint (INP): measures responsiveness. To provide a good user experience, pages should aim for an INP of 200 milliseconds or less.

Unlike FID, which only considered the first interaction, INP evaluates the latency of all interactions, offering a more complete view of page responsiveness.

Testing and monitoring tools

GOOGLE TOOLS

To monitor and optimize Core Web Vitals, Google offers several tools—each with specific functions to help analyze page performance, identify issues, and take action.

• Google Search Console – Report Core Web Vitals: Provides real-world data from the Chrome User Experience Report (CrUX), broken down by mobile and desktop. Useful for identifying groups of URLs with similar issues and tracking performance over time. Accessible in the “Experience” > “Core Web Vitals” section in Search Console.

• PageSpeed Insights: Analyzes individual pages with both field data (CrUX) and lab data (via Lighthouse). Offers detailed suggestions for improvement. Available at: PageSpeed Insights.
• Lighthouse: An open-source tool built into Chrome DevTools and also usable via command line. Provides detailed audits on performance, accessibility, SEO, and best practices. Measures metrics CWV like LCP, CLS, and INP.
• Chrome DevTools: Through the “Performance” and “Lighthouse” panels, allows deep analysis of page loading and user interactions. Useful for identifying bottlenecks and optimizing code.

THIRD-PARTY TOOLS

In addition to Google’s tools, many third-party solutions offer advanced features for testing and monitoring user experience quality. These tools allow deeper analysis, testing from multiple locations, and ongoing performance monitoring.

• Webpagetest: Tests web page loading from various geographic locations, browsers, and devices under real network conditions. Accurately measures LCP, INP, CLS, and other key metrics, with visual aids to pinpoint performance issues. Supports multiple tests, video recordings, page comparisons, and filmstrip views.
• GTMetrix: Offers regional testing, slow connection simulations, and video recordings of page loads. Can send automatic alerts when performance drops below preset thresholds. Uses Lighthouse and Web Vitals API for testing, offering detailed reports with LCP, INP, and CLS values. Helps pinpoint where to improve for better user experience.

For a comprehensive list of over 23 CWV testing tools, visit SEOSLY.

Where does the data come from?

To interpret Core Web Vitals metrics correctly, it’s essential to understand the difference between lab data and field data. These offer two distinct perspectives: one technical and controlled, the other based on real user experiences.

• Field Data: based on real users’ experiences and used by Google for ranking. It reflects actual devices, networks, and browsing conditions.
• Lab Data: collected in controlled environments. Not fully representative of real-world user experience but ideal for technical debugging.

Some tools rely on Field Data, which are collected directly during real user browsing sessions. Examples include:

• Google Search Console (Core Web Vitals section)
• PageSpeed Insights (report section: “Field Data,” if available)
• Chrome User Experience Report (CrUX dataset)

Other tools rely on Lab Data, collected through simulations run in controlled environments. These are some of the tools that simulate page loading under standardized conditions:

• PageSpeed Insights (report section: “Lab Data”)
• Lighthouse (also integrated in Chrome DevTools)
• Chrome DevTools – Performance Panel
• WebPageTest (uses lab data by default, but can also be configured for field data)
• GTmetrix (uses lab data based on Lighthouse).

Key optimization techniques

Below are the elements that affect each CWV metric and best practices to improve performance. Remember: CWV scores are indicative—they help identify potential UX issues. The ultimate goal is not perfect scores but a better user experience.

LCP OPTIMIZATION STRATEGIES

A good LCP should be under 2.5 seconds. Main influencing factors:

• Slow server response times
• Heavy, unoptimized resources (images, fonts, assets)
• Render-blocking JavaScript and CSS

To address issues and improve Largest Contentful Paint (LCP), the following best practices should be followed:

• Apply image compression
• Convert your images to next-gen formats (e.g., WebP)
• Use link rel=preload to prioritize loading of LCP elements
• Minify and compress code files
• Implement Critical CSS
• Eliminate render-blocking resources
• Upgrade server infrastructure
• Use a Content Delivery Network (CDN)
• Maximize caching strategies

CLS OPTIMIZATION STRATEGIES

A good Cumulative Layout Shift (CLS) score should be below 0.1. The main elements affecting CLS include:

• Images and videos without defined dimensions (width and height attributes)
• Ads, embeds, and iframes without reserved space
• Web fonts causing flashes of unstyled or invisible text (FOUT and FOIT)
• Inserting content above existing content

To effectively manage these issues, follow these guidelines:

• Avoid injecting new content at the top of the page above existing elements
• Add width and height attributes to images to prevent layout shifts
• Reserve sufficient space for ads, embeds, and iframes
• Use font-display: swap in the @font-face CSS to control font rendering behavior
• Use link rel=preload for key web fonts to ensure early availability

INP OPTIMIZATION STRATEGIES

A good Interaction to Next Paint (INP) score should be below 200ms. Common causes of poor INP include:

• Long tasks blocking the main thread
• Overly large DOM size
• HTML rendered client-side

To achieve a good INP score and ensure responsiveness, consider the following optimizations:

• Avoid recurring timers that overload the main thread
• Break up long tasks to free the main thread
• Avoid overlapping interactions
• Evaluate and remove unnecessary callbacks
• Defer non-render-critical scripts
• Reduce DOM size

Even the smallest SEO aspects count: when well-optimized, they bring measurable, concrete results. Of course, applying best practices isn’t always straightforward—it often requires significant development effort, so it’s important to start with a cost-benefit analysis.

From experience, we can say that addressing some Core Web Vitals (CWV) aspects that require relatively low technical effort can already yield highly satisfying results. This is especially true for image, font, or caching optimization—elements that are used across all page types and, when properly managed, bring major improvements in page loading times and the end-user experience.

How much do CWVs impact SEO?

To understand how Core Web Vitals impact SEO, we need to look at it from the user’s perspective.
Online users have surprisingly low tolerance for slow-loading websites. According to Google the probability of bounce increases by 32% when page load time goes from 1 to 3 seconds. Furthermore, on mobile, 53% of visits are abandoned if a page takes longer than 3 seconds to load.

John Mueller, one of Google’s leading voices on SEO, says:
“We’ve been pretty clear that Core Web Vitals are not giant factors in ranking, and I doubt you’d see a big drop just because of that. […] Having a website that provides a good experience for users is worthwhile, because if users are so annoyed that they don’t want to come back, you’re just wasting the first-time visitors to your site, regardless of where they come from.”

In other words, Core Web Vitals matter for SEO, but they are neither the only nor the most important ranking factor. Google considers them as ranking signals related to user experience:

• If your site has great content, strong authority, and solid technical SEO, poor Core Web Vitals won’t tank your rankings
• On the other hand, if competition is fierce and content quality is similar across sites, good Core Web Vitals can become a competitive edge

In short, Core Web Vitals can’t make up for poor content or neglected SEO basics—but they enhance the user experience and can help improve rankings. That’s where their real value lies.