What Are Core Web Vitals? A Technical Guide to LCP, INP, CLS, and SEO Core Web Vitals چیست؟ بررسی LCP، INP و CLS و تأثیر آن‌ها بر سرعت و سئو سایت

What Are Core Web Vitals? A Technical Guide to LCP, INP, CLS, and SEO

Building a successful website is not only about creating an attractive design. Users expect pages to load quickly, interactions to respond without noticeable delays, and content to remain visually stable while the page is loading.

This is where Core Web Vitals become important. Core Web Vitals are a set of important user experience metrics that focus on three major aspects of web performance: loading performance, responsiveness, and visual stability.

In this article from Just Programmer, we will explore what Core Web Vitals are, how LCP, INP, and CLS work, what causes poor scores, and how developers can improve website performance.

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What Are Core Web Vitals?

Core Web Vitals are a set of standardized metrics used to measure important aspects of user experience on web pages.

The three current Core Web Vitals are:

• LCP — Largest Contentful Paint: Measures loading performance.
• INP — Interaction to Next Paint: Measures responsiveness to user interactions.
• CLS — Cumulative Layout Shift: Measures visual stability.

These metrics focus on three important parts of the user experience:

Loading
   ↓
LCP

Interaction
   ↓
INP

Visual Stability
   ↓
CLS

The goal of Core Web Vitals is not simply to generate a score. Instead, these metrics help developers understand how users actually experience a web page.

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How Are Core Web Vitals Related to SEO?

Core Web Vitals are connected to page experience and Google's ranking systems. Google states that Core Web Vitals are used by its ranking systems and recommends that website owners maintain good Core Web Vitals.

However, an important misconception should be avoided: excellent Core Web Vitals do not automatically place a website in the first position of Google Search.

Search rankings depend on many different factors. Content relevance, content quality, site structure, helpfulness, and many other signals also matter.

Therefore, Core Web Vitals should not be optimized only to achieve a high SEO score. The main goal should be to provide a fast, responsive, and stable experience for real users.

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What Is LCP? Largest Contentful Paint

LCP stands for Largest Contentful Paint. It measures how long it takes for the largest visible content element in the viewport to be rendered.

This element may be a large image, a text block, or a video.

In simple terms, LCP answers this question:

“How quickly does the main content of the page become visible to the user?”

For example, if a page has a large Hero image and that image is the largest visible element, its rendering can have a significant impact on LCP.

LCP Thresholds

For evaluating the experience of most users, the 75th percentile of page loads is commonly used, segmented by mobile and desktop devices. A good LCP target is 2.5 seconds or less. :contentReference[oaicite:13]{index=13}

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What Causes a High LCP?

A poor LCP can have many different causes, and the problem is not always simply a large image.

• Slow server response
• High TTFB
• Large or unoptimized images
• Heavy CSS files
• Slow-loading web fonts
• Render-blocking resources
• Heavy JavaScript
• Unnecessary redirects
• Slow network conditions
• Poor resource loading strategies

How to Improve LCP

• Compress large images and use WebP or AVIF when appropriate.
• Give important above-the-fold images appropriate loading priority.
• Reduce unnecessary CSS.
• Identify and optimize render-blocking resources.
• Improve server response time.
• Use caching and CDNs when appropriate.
• Optimize font loading.
• Move unnecessary JavaScript away from the critical loading path.

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What Is INP? Interaction to Next Paint

INP stands for Interaction to Next Paint and measures how responsive a page is to user interactions.

An interaction may include clicking a button, tapping an element, selecting an option, or performing another action on a page.

In simple terms, INP measures how quickly the browser can present a visual response after a user interaction.

INP replaced FID as a Core Web Vital in 2024. :contentReference[oaicite:14]{index=14}

INP Thresholds

An INP of 200 milliseconds or less is considered good. The 75th percentile is commonly used for evaluating user experience across mobile and desktop devices. :contentReference[oaicite:15]{index=15}

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Why Can INP Be Poor?

One of the most common causes of poor INP is long-running JavaScript work on the browser's Main Thread.

Imagine that a user clicks a button while the browser is simultaneously executing a heavy JavaScript task. Until the browser can process the work, the visual response to the user's interaction may be delayed.

User Interaction
      ↓
JavaScript Event
      ↓
Long Task
      ↓
Main Thread Busy
      ↓
Delayed Visual Update
      ↓
Poor INP

How to Improve INP

• Reduce unnecessary JavaScript.
• Break long tasks into smaller tasks.
• Reduce expensive calculations inside event handlers.
• Optimize event listeners.
• Use Web Workers for suitable heavy processing.
• Reduce unnecessary JavaScript dependencies.
• Avoid expensive work immediately after user interactions.

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What Is CLS? Cumulative Layout Shift

CLS stands for Cumulative Layout Shift and measures the visual stability of a page.

Have you ever tried to click a button, only for an image or advertisement to load and suddenly move the button somewhere else? This is an example of a poor experience caused by layout shifts.

CLS measures the amount of unexpected visual movement that occurs during a page experience.

CLS Thresholds

A good CLS target is 0.1 or less, with the 75th percentile commonly used to evaluate the experience of most users. :contentReference[oaicite:16]{index=16}

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What Causes Poor CLS?

One of the most common causes of CLS is failing to reserve space for elements before they load.

For example:

<img src="hero.webp" alt="Hero Image">

In this case, the browser may not know the exact space required by the image before it loads.

A better approach is to define the image dimensions:

<img
    src="hero.webp"
    width="1200"
    height="700"
    alt="Hero Image"
>

Other common causes include:

• Images without defined dimensions
• Advertisements and iframes without reserved space
• Dynamically injected content
• Late-loading web fonts
• Elements changing size after rendering
• Unexpected banners or notifications

Images without dimensions, advertisements, iframes, dynamically injected content, and web fonts are among the common causes of layout shifts. :contentReference[oaicite:17]{index=17}

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LCP vs INP vs CLS

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What Is the Difference Between Field Data and Lab Data?

When measuring website performance, you may encounter two major types of data: Field Data and Lab Data.

What Is Field Data?

Field Data is collected from real users under real-world conditions. Device capabilities, network conditions, browser behavior, and other environmental factors can affect the results.

For this reason, Field Data provides a more realistic picture of how users experience a website.

What Is Lab Data?

Lab Data is generated in a controlled testing environment with predefined conditions. It is very useful for identifying technical problems and testing performance improvements.

As a result, a page can perform well in a laboratory test while real users experience different results on different devices and networks.

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What Is PageSpeed Insights?

PageSpeed Insights is a popular tool for analyzing web page performance. It can provide information about page performance and user-experience-related metrics.

For developers, PageSpeed Insights can be a useful starting point for identifying performance problems, but it should not be the only source of performance analysis.

Real-user data should also be considered when evaluating website performance.

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Core Web Vitals in Google Search Console

Google Search Console also provides a Core Web Vitals report that helps website owners understand the performance of their pages.

The report can be especially useful for identifying groups of pages that share performance problems.

Instead of focusing only on one URL or one number, developers should look for recurring patterns across their pages.

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A Real-World Performance Analysis Example

Imagine that a page produces the following results:

LCP = 4.8s
INP = 280ms
CLS = 0.32

All three metrics need improvement.

We can investigate the problems step by step.

Step 1 — LCP

Suppose the Hero image is extremely large. We can optimize it:

Large Image
    ↓
Compression
    ↓
WebP / AVIF
    ↓
Better Loading Priority
    ↓
Lower LCP

Step 2 — INP

If heavy JavaScript is blocking the Main Thread, we can identify Long Tasks and reduce unnecessary work.

Heavy JavaScript
      ↓
Long Tasks
      ↓
Main Thread Blocked
      ↓
Interaction Delay
      ↓
Optimize JavaScript
      ↓
Better INP

Step 3 — CLS

If images, advertisements, or dynamic elements appear without reserved space, we should reserve the required space before those elements load.

Undefined Element Size
        ↓
Layout Shift
        ↓
Higher CLS

Defined Space
        ↓
Stable Layout
        ↓
Lower CLS

Therefore, performance optimization is usually not a single-step task. It is a combination of many technical improvements.

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How Can You Improve Core Web Vitals?

• Compress images and use appropriate formats.
• Define image dimensions.
• Reduce unnecessary CSS and JavaScript.
• Identify and optimize Long Tasks.
• Improve server response time.
• Use caching where appropriate.
• Use a CDN when it makes sense for the project.
• Optimize font loading.
• Reserve space for dynamic content.
• Monitor real-user performance data.
• Test pages on both mobile and desktop devices.

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Common Core Web Vitals Optimization Mistakes

Focusing Only on Lighthouse Scores

A good laboratory score does not mean that every real user will experience the page in exactly the same way. Real-world conditions vary significantly.

Thinking Core Web Vitals Is the Only SEO Factor

Core Web Vitals are important, but Google Search rankings are not determined by a single metric.

Optimizing Only for Desktop

Many users browse the web on mobile devices, and page performance can vary significantly across devices and networks.

Focusing Too Much on One Number

The final goal should be a fast and usable website, not simply achieving a specific score.

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Core Web Vitals Quick Checklist

• ☐ LCP is within the recommended range.
• ☐ INP is within the recommended range.
• ☐ CLS is low.
• ☐ Images have defined dimensions.
• ☐ Large images are optimized.
• ☐ Unnecessary JavaScript has been reduced.
• ☐ Long Tasks have been investigated.
• ☐ Server response time has been optimized.
• ☐ Mobile performance has been tested.
• ☐ Real-user data has been considered.

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Frequently Asked Questions About Core Web Vitals

What metrics are included in Core Web Vitals?

The current Core Web Vitals are LCP for loading performance, INP for responsiveness, and CLS for visual stability. :contentReference[oaicite:18]{index=18}

Do Core Web Vitals automatically make a page rank first on Google?

No. Core Web Vitals are used by Google's ranking systems, but good Core Web Vitals alone do not guarantee top rankings. :contentReference[oaicite:19]{index=19}

What is a good LCP?

An LCP of 2.5 seconds or less is considered good. Values above 4 seconds are considered poor. :contentReference[oaicite:20]{index=20}

What is a good INP?

An INP of 200 milliseconds or less is considered good. Values above 500 milliseconds are considered poor. :contentReference[oaicite:21]{index=21}

What is a good CLS?

A CLS of 0.1 or less is considered good, while values above 0.25 are considered poor. :contentReference[oaicite:22]{index=22}

Is FID still a Core Web Vital?

No. INP replaced FID as the Core Web Vital used to measure responsiveness. :contentReference[oaicite:23]{index=23}

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Conclusion

Core Web Vitals are important metrics for understanding how real users experience a web page. They focus on three major areas: loading performance through LCP, responsiveness through INP, and visual stability through CLS.

Improving these metrics is not only about search engines. It also helps developers build websites that feel faster, smoother, and easier to use.

However, Core Web Vitals should not be treated as a shortcut to the top of Google Search. The strongest approach combines performance with helpful content, good website structure, security, accessibility, and an overall high-quality user experience.

A fast website is not built only for Google; it is built for the people who use it every day.

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