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Headphones website homepage shown on desktop and mobile. Also shown are core web vital measurements of the site.

An in-depth guide to Core Web Vitals (CWV)

Beginning in June, Core Web Vitals (CWV), a new set of performance assessments, took its place in Google's search algorithm. This initiative provides a unified guidance and standardization to help sites deliver better user experiences on the web. As this change rolls out, marketers and site owners need to learn how to leverage these new metrics to deliver faster-loading sites with great user experiences.

At Wix, we are continually working to improve the performance of sites hosted on our platform—to ensure that our sites meet industry standards including CWV.

In this article, we’ll take a closer look at CWV, along with a few other key performance measures, to understand what they mean for your site performance and visitor experience.


Why is user experience so important?

Your visitors come to your site ready to explore your offering and the content you've created for them—but they'll never enjoy what you've built if it takes too long to load. Users will bounce from even the most captivating experience if they're stuck waiting for your site to load on their devices.

That’s one reason why Google is now emphasizing website performance (e.g. speed) as a ranking factor on its search engine. When the primary ranking factors, such as content and authority, are very close, site speed can act as a tiebreaker. As a result, good performance can impact rankings in search results.

As Core Web Vitals become integral to Search Engine Optimization (SEO), site owners are looking to improve their CWV assessments. Though it won't replace other important measurement factors, its relative simplicity, transparency and actionable insights make CWV an excellent resource for understanding and monitoring your site's performance.

Understanding Core Web Vitals

Core Web Vitals is a new set of assessments based on field data (anonymized data collected from actual visits to your sites) designed to help standardize and simplify how site owners monitor three key factors that impact visitor experience the most: loading time, interactivity and visual stability. And because these metrics are based on actual user data, CWV scores are fluid and change as users interact with your site.

Google's tools present CWV as both numerical scores and simple labels, so you can see at a glance how well your site is performing.

  • “Good” CWV scores mean everything is happening quickly and smoothly. This means your user experience is doing just fine. 

  • “Needs improvement” CWV scores alert you to issues that may be slowing down your site.

  • “Poor” CWV scores warn when your user experience (UX) isn’t up to par with industry standards.


Core Web Vitals assesses three essential UX aspects that affect how fast your site performs:


  • Loading: Largest Contentful Paint (LCP) measures the time from when the page started loading, until the render of the largest image or text block visible within the initial viewport. For a good score, LCP must be 2.5 seconds or less.


  • Interactivity: First Input Delay (FID) measures the time from when a user first interacts with a page, e.g. by clicking on a link or button, until the browser is actually able to process that interaction. For a good score, FID must be 100 milliseconds or less.

  • Visual Stability: Cumulative Layout Shift (CLS) measures the total of all individual, unexpected layout shifts that occur during the entire lifespan of the page.  An unexpected layout shift occurs any time a visible element changes its position without user interaction. For a good score, CLS must be 0.1 or less.

In addition to CWV, Page Experience includes other aspects of user experience that are not about performance. These are:


  • Mobile-friendliness: Users should be able to read and use the mobile-friendly version of your site without having to pinch or zoom in to read content.

  • Safe Browsing: Google uses Safe Browsing to ensure that sites don’t contain “malicious” or “deceptive” content. You can test your site with a Security Issues report.

  • HTTPS: Your site’s safety and security—both for you and your visitors—is paramount, and search engines take this into account. HTTPS uses an encryption protocol and is more secure than serving your site over HTTP. Good news: All Wix sites are HTTPS. 

  • Intrusive interstitials: By law, websites must inform users about things like data collection and privacy policies by displaying interstitials, but intrusive interstitials can hinder content accessibility.

Explanation of the 3 core web vitals: LCP, FID and CLS. And a description of how they are measured.
Factors that impact a site's user experience and ranking including loading time, interactivity, visual stability and more.

The CWV metrics that Google uses for the Page Experience ranking is based on field data only, and does not use any lab data or other simulated tests. This field data is also known as Real User Monitoring (RUM) because it’s collected from the sessions of real users.


CWV, as well as additional data, is collected from every Chrome session that users haven’t opted out of. This data is sent to Google servers, and stored in a database known as Chrome User Experience (CrUX). The Google search engine will then query this database in order to compute the Page Experience ranking signal. The CrUX data is also accessible and presented by tools such as Google PageSpeed Insights (PSI), and the Google Search Console itself.


In addition to the field data, Google also provides tools for testing performance of specific sessions under lab conditions, including Google Lighthouse and PageSpeed Insights (which we will cover later on). There are also lab tools from other sources, such as WebPageTest and GTmetrix. These are especially useful for quickly understanding possible changes that may impact performance, or to test sites that don't yet have sufficient field measurements.

How does Google collect Core Web Vitals field data (and what’s the deal with lab data?)

Chart showing the strengths and limitations of Google performance field tools and lab tools.

Strengths and limitations: Field vs. lab tools

The three metrics that measure Core Web Vitals

Understanding Core Web Vitals (CWV)

Tools for measuring your site’s Core Web Vitals

To better integrate the insights of Core Web Vitals with Google’s suite of measurement tools, Google is now incorporating CWV metrics into its other popular measurement tools, including some that have historically focused on reporting lab data. Here’s how and where to find your CWV assessments:


Field Tools:

  • Google Search Console Core Web Vitals report: Naturally, your first stop for Core Web Vitals is your Core Web Vitals report in the Google Search Console (GSC), which estimates how quickly your pages load over any given period. This report analyzes all the URLs on your site that Google has indexed, checks their scores against Core Web Vitals data and puts these into buckets, with separate scores for desktop and mobile. As noted in the earlier section, the three core metrics reported by Core Web Vitals are Largest Contentful Paint (LCP), First Input Delay (FID) and Cumulative Layout Shift (CLS).

    *If Search Console says "No Data Available" then either your property is new to Search Console or there is not enough data in CrUX report that can be pulled into Search Console. 


  • Chrome User Experience (CrUX): The Chrome User Experience Report is powered by real user data measurements of key UX metrics across the public web. The data is aggregated from users who haven’t disabled their usage reporting. You can set up the CrUX Dashboard on Google Data Studio to track an origin’s UX trends.


Lab Tools:

  • Google Lighthouse: Google Lighthouse audits for performance, accessibility, progressive web apps (PWAs), SEO and more. Along with Core Web Vitals LCP and CLS (as well as Total Blocking Time - TBT), Lighthouse measures the following lab data metrics:

    • First Contentful Paint (FCP): FCP marks the first point during page loading when users see something on their screens. A fast FCP can help reassure users that something is happening when they land on your site. A slow FCP means that your users aren't seeing anything right off the bat (and that's not good).

    • Speed Index: Speed Index measures how quickly content is visually displayed during page load. This is critical for a great user experience.

    • Time to Interactive (TTI): TTI measures how long it takes for a page to become fully interactive. For example, TTI will tell you how much time it takes after the page loads before a user can click a button.


  • Google PageSpeed Insights: PageSpeed Insights (PSI) is a Google tool that provides both lab and field data about a page and assesses page speed; it now also incorporates all three CWV metrics. When analyzing a URL, you'll receive two separate reports—one for desktop and one for mobile—and these two scores will likely be different because:

    • Mobile devices generally have slower processors than desktops.

    • Mobile devices need to resize images and site elements to fit the viewport.

    • PageSpeed Insights displays the site loading time on mobile with a simulated 3G connection.


  • GTmetrix: GTmetrix uses a combination of Google PageSpeed Insights and YSlow to generate scores and recommendations. The tool generates summary, performance, structure and waterfall reports. 



Remember that a site's performance can vary dramatically based on a user's device capabilities, network conditions and other processes running on the device. As noted earlier, because Core Web Vitals metrics are field data drawn from actual user interactions with your site, your Core Web Vitals assessments will fluctuate more than the lab data measured by lab tools.

What Google says about CWV and SEO

In the latest Google I/O conference in May 2021, Google provided some clarifications on how they plan on using CWV for SEO. 


I. Initially, the performance ranking boost will be only for mobile. Desktop support will be added later on (no timeframe was provided).

II. It's only a boost, not a degrading factor. That said, a site’s page rank could decrease if its competitors get a boost and it doesn't.

III. Content is still king, and will always be. Content quality, and authority, have a much higher ranking impact than performance.

IV. You don't need to get good scores for all three CWV in order to get the performance ranking boost: each metric will contribute a boost independently.

V. You don't need to get a good (green) score for any CWV to get a boost: you get zero boost for a poor (red) metric score. It then starts increasing gradually in the yellow zone, and plateaus when it reaches green.


That being said, having good performance has always been beneficial for a page's success, and will continue to be so going forward.

Learn more from Google’s Q&A session.

Measuring CWV

How Wix is getting ready for Core Web Vitals

To improve performance for the millions of sites created on our platform, we've implemented several new tools and strategies in preparation for Google incorporating CWV into its page ranking. These include: 

  • Launching our new Site Speed Dashboard to help users monitor their site performance with real-time metrics like loading time (from field data) and tips for improving page speed. Additionally, the dashboard provides PageSpeed Insight scores, based on lab data, for both mobile and desktop. 

  • Reducing website rendering time both desktop and mobile. 

  • Caching rendered pages and static resources in a global CDN for fast delivery.

  • Offloading computations for the endpoint devices to our fast server.


Through these and other efforts, we grew the number of Wix sites with a good score by 8x.

How to improve Core Web Vitals results on your Wix site

If your Core Web Vitals are not up to par, it’s important to troubleshoot your site to improve your performance. Here’s what you can do to improve your CWV scores:


1. Avoid overloading pages with excessive content.

2. Break long pages into multiple shorter pages, focused on specific content or functionality.

3. Use JPGs over PNGs and videos instead of GIFS where possible. 

4. Avoid excessive use of different fonts or font-weights.

5. Reduce use of animations—reveal animations in particular.

6. Don't put your primary content inside lightboxes.

To learn more about best practices for improving your site for CWV, check out our article on how to optimize your website for better performance.


Awareness is the first step on the path to success, so always monitor your Core Web Vitals (along with the other elements we’ve covered)—and then nip any problems you see in the bud by regularly optimizing your site for better performance.

Graph showing the significant improvement of performance of Wix sites since the end of 2020, as measured by Core Web Vitals.

Optimizing for CWV on Wix


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Knowing when to upsell a client can feel more art than science, but a study by Vendasta claims to have found the perfect time to expand your relationship with a client: 3 months.


Ninety days may seem like a short timeline to start upselling, but if a client is satisfied with your work in that time period then upselling can bring them more value quicker than they expected. Alternatively, if they’re not satisfied, it gives you a chance to recalibrate so you can keep their business. Both of those scenarios result in additional revenue you would have missed out on had you not initiated the conversation.


Upsells are easier to approach and able to start earlier than most agencies realize. Here are four critical steps that will help you upsell sooner successfully.

Dan focuses on optimizing all sites hosted on Wix for speed. With over 25 years of software development experience, Dan has worked on systems ranging from multi-user games to missile trajectory simulations. He is a frequent speaker at technical conferences, a panelist on the JavaScript Jabber podcast, and a member of the Google Chrome Advisory Board (CAB). Dan holds an M.Sc. in Computer Science.

Author: Dan Shappir

Performance Tech Lead at

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Author: Dan Shappir

Dan focuses on optimizing all sites hosted on Wix for speed. With over 25 years of software development experience, Dan has worked on systems ranging from multi-user games to missile trajectory simulations. He is a frequent speaker at technical conferences, a panelist on the JavaScript Jabber podcast, and a member of the Google Chrome Advisory Board (CAB). Dan holds an M.Sc. in Computer Science.

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