What are Core Web Vitals? Explanation of the impact on SEO and improvement methods for the 3 indicators



  • “I know that Core Web Vitals are important.”

    “However, I don’t understand them deeply enough to work on improvements for my own site…”

    Even web administrators and SEO specialists might find themselves in a situation similar to the above, don’t you think?

    “Core Web Vitals” refer to three speed metrics that Google considers crucial for achieving an excellent user experience.

    According to Google’s research, pages that meet the three criteria of Core Web Vitals had a 24% lower chance of users abandoning the page during loading.

    Improving Core Web Vitals not only provides a stress-free and pleasant experience for page visitors but also has positive effects on SEO.

    This article summarizes Core Web Vitals to give beginners an overview.

    Topics in This Article

    • What are Core Web Vitals
    • Impact of Core Web Vitals on SEO
    • Why are Core Web Vitals important
    • Criteria for evaluation and priority of Core Web Vitals improvement
    • Methods to check and tools for measuring Core Web Vitals
    • Ways to improve Core Web Vitals
    • Other web vital indicators

    If you haven’t touched Core Web Vitals at all, please consider working on improvements.

    What are Core Web Vitals: The 3 Metrics “LCP,” “FID,” and “CLS”

    Core Web Vitals (CWV) are UX metrics introduced by Google to quantify the “quality of user experience.”

    They represent the comfort and operability users feel when using a web page.

    Specifically, the three metrics – “LCP,” “FID,” and “CLS” – are collectively referred to as “Core Web Vitals.”

    ※ As of now, these three metrics are “LCP,” “FID,” and “CLS,” but Google has announced that “FID” will be replaced by “INP” in March 2024 (Google announcement).

    Core Web Vitals are also described in Japanese as “main indicators related to the web.”

    To deeply understand Core Web Vitals, it’s necessary to be familiar with the concept of “web vitals.”

    “Web Vitals” is an initiative by Google with the following objective:

    • Create common guidelines for all site operators to provide an excellent user experience

    The three metrics of “Core Web Vitals” are specific indicators that Google has concretely proposed in this effort to improve web vitals.

    3 Metrics of Core Web Vitals

      LCP: Metric measuring loading speed (How fast does content appear on the screen?)
      FID: Metric measuring interactivity (How fast does the page respond to interactions?)
      CLS: Metric measuring visual stability (Does the layout shift occur during loading?)

      LCP (Largest Contentful Paint)

      LCP is a metric designed to measure “page loading speed.”

      It indicates the time it takes for the largest content on the page (such as images, videos, text blocks) to appear on the screen.

      LCP translates to “Largest Contentful Paint.”

      The value is expressed in “seconds,” and a smaller value indicates a better user experience.

      FID (First Input Delay)

      FID is a metric for measuring “interactivity (responsiveness).”

      It represents the time it takes for the browser to respond after the user’s initial action (click, tap, text input, etc.).

      For example, if there is a delay in the ability to interact with the page even after it has loaded, the FID is not optimal.

      FID translates to “First Input Delay.”

      The value is expressed in “milliseconds,” and a smaller value indicates a better user experience.

      CLS (Cumulative Layout Shift)

      CLS is a metric for measuring “visual stability.”

      It indicates how much unexpected layout shifts occur.

      Unexpected layout shifts, for example, occur when:

      • Trying to read an article, and images or ads appear late, pushing the main text downward
      • Attempting to click a button, and the layout shifts, causing a click on a different button

      These are instances where page elements move during loading.

      CLS translates to “Cumulative Layout Shift.”

      The value is expressed as a score (unitless), and a smaller value indicates a better user experience.

      [Scheduled for Adoption in March 2024] INP (Interaction to Next Paint)

      Google recently announced the replacement of “FID” with “INP” as one of the three metrics of Core Web Vitals.

      The replacement is scheduled for March 2024.

      The “INP” metric itself has been in trial use since May 2022.

      Similar to FID, INP is a metric indicating responsiveness, showing the time it takes for a visual reaction to occur after a user’s interaction.

      While FID measures “input delay for the first input,” INP measures “interaction across the entire range that occurs from page loading to departure.”


      The indicators constituting Core Web Vitals are not fixed. There is a possibility of replacement with other indicators in the future.
      Google explains, “The indicators constituting Core Web Vitals evolve over time.”
      Reference: Web Vitals

      Impact of Core Web Vitals on SEO

      The scores of Core Web Vitals directly influence SEO.

      Regarding the relationship between Core Web Vitals and SEO, here are two important points:

      • Core Web Vitals are ranking factors
      • Their influence on rankings is relatively small

      Let’s delve into the details.

      Core Web Vitals are Ranking Factors

      As Core Web Vitals are ranking factors (i.e., factors determining rankings), they directly impact rankings.

      They were introduced in mobile searches in June 2021 and in PC searches in February 2022.

      Core Web Vitals are incorporated into ranking factors as one of the “Page Experience” metrics.

      Page Experience metrics represent “how users perceive the operability of a web page.”

      As of now, the “Page Experience” metrics influencing rankings include the following four:

      Page Experience Metrics

      • Core Web Vitals
      • Mobile-Friendly
      • HTTPS
      • No intrusive interstitials (excessive pop-up ads, etc.)

      Source: Evaluating Page Experience for a Better Web | Google Search Central Blog

      Influence on Rankings is Relatively Small

      While Core Web Vitals are one of the ranking factors, their impact is not as significant as “quality” or “relevance.”

      Core Web Vitals have a substantial impact on rankings in situations where the competition is close.

      Google explains the impact on rankings as follows:

      While page experience is important, Google still strives to rank pages with overall valuable information even if the page experience is subpar. In other words, no matter how excellent the page experience is, it won’t surpass a page with excellent content. However, in search rankings where there are numerous pages with similar relevance, page experience becomes even more crucial.

      Source: Page Experience and Its Impact on Google Search Results | Google Search Central

      In other words, Core Web Vitals come into play in situations where it’s challenging to differentiate content quality and relevance.

      In terms of the weight as a ranking factor, “content quality” and “relevance” far outweigh Core Web Vitals. Even if all Core Web Vitals are excellent, you cannot rank high with low-quality or irrelevant content.

      Why are Core Web Vitals Important? [Benefits of Improvement]

      However, the benefits of improving Core Web Vitals go beyond SEO, so web administrators should strive for improvement.

      The smooth operation of pages should be a necessity to achieve your ultimate goal (delivering content to users).

      Have you ever experienced frustration with a page that loads slowly or has poor interactivity, ultimately closing it without viewing the content?

      Google has shared the following data:

      • 50% of users abandon a site if it takes more than 3 seconds to load.
      • 52% of users consider loading time important for determining whether they like a brand or not.

      Source: Maintaining Visitor Engagement with Key Web Metrics – Google News Initiative

      In this way, the loading speed actually affects whether users can thoroughly view the content and form a positive impression of the site or company.

      By improving Core Web Vitals, your pages are less likely to be abandoned, leading to increased conversion rates.

      In reality, the following companies have reported positive impacts on user behavior and revenue after implementing improvements:

      • Netzwelt improved key web metrics, resulting in an 18% increase in ad revenue and a 27% increase in page views.
      • For each 100 ms reduction in Largest Contentful Paint (LCP), Farfetch’s web conversion rate increased by 1.3%.
      • By reducing Cumulative Layout Shift (CLS) by 0.2, Yahoo! JAPAN experienced a 15% increase in page views per session, a 13% longer session duration, and a 1.72% decrease in bounce rate.

        Source: Key Web Metrics Report – Search Console Help

      Criteria for Judging Core Web Vitals and Priority for Improvement

      Core Web Vitals have criteria values set as follows:

      Criteria for Judging Core Web Vitals
       GoodNeeds ImprovementPoor
      LCPBelow 2.5 secondsBelow 4 secondsExceeds 4 seconds
      FIDBelow 100 millisecondsBelow 300 millisecondsExceeds 300 milliseconds
      CLS0.1 or Below0.25 or BelowExceeds 0.25

      Reference: Key Web Metrics Report – Search Console Help

      Each value can be evaluated on a three-point scale: “Good,” “Needs Improvement,” and “Poor.”

      Although the units are different, smaller values indicate a better user experience.

      The priority for improving Core Web Vitals is as follows:

      Priority for Improving Core Web Vitals

      1. “Poor” → Needs urgent improvement. Has a significant impact on both rankings and UX.
      2. “Needs Improvement” → Improvement is desirable. Even a slight improvement in scores may enhance SEO ratings.
      3. “Good” → Scores won’t improve further, and there is no impact on SEO ratings (low priority). However, it is effective for further improving user experience.

      Reference: Core Web Vitals are Gradual Evaluation, Not Binary – Overseas SEO Information Blog

      Start by prioritizing the improvement of “Poor” pages.

      Methods of Checking and Measurement Tools for Core Web Vitals

      Representative tools for checking the values of Core Web Vitals are the following four:

      Methods of Checking and Measurement Tools for Core Web Vitals

      1. “Search Console”
      2. “PageSpeed Insights”
      3. Chrome Extension “Web Vitals”
      4. Chrome Extension “Lighthouse”

      Since each tool has its own features, we will explain how to use them along with their advantages and disadvantages.

      If you’re unsure which tool to use, it’s generally recommended to follow these steps:

      1. Check and identify problematic pages using “Search Console”
      2. Check individual page scores and improvement points using “PageSpeed Insights”

      1. “Search Console”

      Google Search Console automatically detects pages with Core Web Vitals issues across the entire site you operate.

      The confirmation process is as follows:

      1. Click on “Core Web Vitals” on the left side of the management screen to display reports for both “Mobile” and “PC”.

      2. Click on “Open Report” to see a breakdown of pages judged as “Poor,” “Needs Improvement,” or “Good,” and check which of the “LCP,” “FID,” and “CLS” metrics have issues.

      However, please note that if the data volume does not meet the criteria, it may not be reflected in the report.

      Features of Search Console
      • Can check pages with issues across the entire site
      • Cannot confirm detailed fixes or causes for individual pages

      2. “PageSpeed Insights”

      If you want to check and improve Core Web Vitals for individual pages, PageSpeed Insights is recommended.

      It provides detailed information, including not only the numerical values of each metric but also hints for improvement.

      Simply access the link above and enter the target URL to use it easily (no registration required).

      PageSpeed Insights provides both “Field Data” (real data obtained when users access) and “Lab Data” (simulation results).

      • Field Data (data obtained by actual users) → “Evaluate in actual user environment”
      • Lab Data (simulation results) → “Diagnose performance issues”

      Since Lab Data is a simulation result, you can judge Core Web Vitals even without actual access (for example, for pages just published that day).

      Features of PageSpeed Insights
      • Can check numerical values and improvement methods for each individual page in detail
      • Both Field Data and Lab Data can be confirmed
      • Can only be confirmed for each individual page

      3. Chrome Extension “Lighthouse”

      Lighthouse is a site audit tool for developers provided as a Chrome extension.

      However, Lighthouse only allows you to check lab data (simulation results).

      Also, since the “FID” score is not displayed, you need to refer to the metric called “TBT” instead.

      By following the link above and clicking on “Add to Chrome” and then “Generate report,” you can create a report.

      Features of Lighthouse
      • Detailed check of numerical values and improvement methods (lab data) for each individual page
      • Cannot check field data or the value of “FID”

      4. Chrome Extension “Web Vitals”

      Web Vitals is also provided as a Chrome extension.

      You can measure core web vital metrics for PC pages in real-time while browsing.

      It is very useful for simple measurements and checking competitor pages.

      By following the link above, you can use it by clicking on “Add to Chrome.”

      Features of Web Vitals
      • Easy judgment on the page itself
      • May interfere with other Chrome extensions and provide inaccurate values

      Improvement Methods for Core Web Vitals

      Here, we introduce possible factors and improvement methods when the scores for “LCP,” “FID,” and “CLS” are low.

      1. Improvement methods for LCP

      There are various reasons for a decrease in LCP, but the following factors are generally considered:

      Factors causing a decrease in LCP

      • Time-consuming loading of images or videos
      • Long server response time
      • JavaScript and CSS hindering rendering
      • Rendering on the client side

      To improve LCP, you can consider the following methods:

      Improvement methods for LCP

      • Reduce the size of images (resize, compress, change file format, delete if unnecessary)
      • Implement lazy loading of images (i.e., not loading elements that are not visible in the first view)
      • Review server specifications, use Content Delivery Network (CDN)
      • Remove unused CSS and JavaScript
      • Perform rendering on the server side

      Google’s advice on improving LCP can be found on the following page:
      Optimize Largest Contentful Paint

      2. Improvement methods for FID

      The main factor causing a decrease in FID is “JavaScript.”

      Factors causing a decrease in FID

      • Execution of heavy JavaScript

      To improve FID, you need to take the following measures:

      Improvement methods for FID

        Remove unnecessary JavaScript
        Minimize and compress JavaScript
        Implement Web Workers (tools that execute JavaScript in the background)

        Google’s advice on improving FID can be found on the following page:
        Optimize First Input Delay

        3. Improvement methods for CLS

        Common factors causing a decrease in CLS include the following:

        Factors causing a decrease in CLS

        • Images or ads without specified sizes
        • Embedding of dynamic content
        • Loading of web fonts

        The improvement methods for CLS are as follows:

        Improvement methods for CLS

        • Specify image sizes in advance
        • Pre-allocate space for ads, change their positions
        • Avoid using web fonts as much as possible, and if used, prioritize loading

        Google’s advice on improving CLS can be found on the following page:
        Optimize Cumulative Layout Shift

        If the scores for each metric are not good, please try the improvement methods mentioned above.

        Other Web Vital Metrics

        Metrics provided by tools like “PageSpeed Insights” from Google are not limited to the three core web vital metrics (LCP, FID, CLS).

        Other web vital metrics are also measured.

        Let’s briefly introduce each of them:

        • FCP (First Contentful Paint)
          An indicator representing the time until some element is displayed on the screen.

        • TTFB (Time to First Byte)
          An indicator representing the response speed of the web server.

        • SI (Speed Index)
          An indicator representing the speed at which content is visually displayed during page loading.

        • TBT (Total Blocking Time)
          An indicator representing the total time user interactions such as clicks or inputs are blocked. In tools that measure scores based on lab data, TBT is referred to instead of FID.

        [Summary] Let’s Improve Core Web Vitals for Site Visitors

        Core Web Vitals are metrics introduced by Google with the aim of improving the quality of user experience.

        Consisting of the three elements “LCP,” “FID,” and “CLS,” they have also become ranking factors in Google search since 2021.

        In SEO, where there are very few measurable indicators, Core Web Vitals are among the few measurable metrics.

        Improving Core Web Vitals is essentially for the users visiting your site, and as user experience improves, positive impacts on conversions and revenue can be expected.

        Let’s work on improving Core Web Vitals to create a user-first, high-quality site.