How to Optimize Your Website to Pass Core Web Vitals

What are Core Web Vitals?

Core Web Vitals (CWV) are a set of specific metrics that Google uses to measure the real-world user experience of a webpage, focusing on loading performance, interactivity, and visual stability. Passing the Core Web Vitals assessment is important not only for providing a good user experience but also as a ranking factor in Google Search.

The three core metrics are: 1. Largest Contentful Paint (LCP): Measures loading performance. It marks the point in the page load timeline when the main content of the page has likely loaded. 2. First Input Delay (FID): Measures interactivity. It quantifies the experience users feel when trying to interact with a page that isn't yet responsive. 3. Cumulative Layout Shift (CLS): Measures visual stability. It quantifies how much the content on a page unexpectedly shifts around during the loading process.

To pass the assessment, your page must meet the "Good" threshold for all three metrics for at least 75% of its real-world users over a 28-day period.

Metric Good Needs Improvement Poor
LCP ≤ 2.5 seconds > 2.5s and ≤ 4.0s > 4.0 seconds
FID ≤ 100 milliseconds > 100ms and ≤ 300ms > 300ms
CLS ≤ 0.1 > 0.1 and ≤ 0.25 > 0.25

How to Improve Largest Contentful Paint (LCP)

LCP is all about how fast the largest element (usually a hero image, video, or a large block of text) becomes visible to the user.

1. Optimize Your Server Response Time (TTFB):

  • Use a CDN: A Content Delivery Network (CDN) caches your content at edge locations closer to your users, dramatically reducing network latency.
  • Enable Caching: Implement proper server-side and browser caching to serve content faster.
  • Upgrade Your Hosting: A slow server will always result in a slow Time to First Byte (TTFB).

2. Optimize Your Images:

  • The LCP element is often an image. Ensure it's properly optimized.
  • Compress Images: Use tools like ImageOptim or online services to reduce the file size of your images without sacrificing too much quality.
  • Use Modern Image Formats: Serve images in next-gen formats like WebP or AVIF, which offer better compression than JPEG or PNG.
  • Use Responsive Images: Use the <picture> element or the srcset attribute to serve different image sizes for different screen resolutions. Don't serve a massive desktop image to a mobile user.

3. Defer Offscreen Images (Lazy Loading):

  • Use native lazy loading (<img loading="lazy" ...>) for all images that are "below the fold" (not immediately visible). This allows the browser to prioritize loading the critical, visible content first.

4. Preload the LCP Image:

  • If you know which image will be your LCP element, you can tell the browser to prioritize loading it by adding a preload hint in the <head> of your HTML: <link rel="preload" as="image" href="/path/to/hero-image.webp">

How to Improve First Input Delay (FID)

FID measures the time from when a user first interacts with your page (e.g., clicks a button) to the time the browser is actually able to process that event. A long FID is usually caused by the browser's main thread being busy parsing and executing large JavaScript files.

1. Break Up Long Tasks:

  • If you have a single, large JavaScript file that takes a long time to execute, it will block the main thread. Use code splitting to break it up into smaller, asynchronous chunks that can be loaded on demand.

2. Defer or Async JavaScript:

  • Use the defer or async attributes on your <script> tags.
    • async: Downloads the script without blocking the HTML parser and executes it as soon as it's downloaded.
    • defer: Downloads the script without blocking and executes it only after the HTML parser has finished. defer is generally preferred as it maintains the order of execution.

3. Minimize Third-Party Scripts:

  • Third-party scripts for analytics, ads, or social media widgets can be heavy and block the main thread. Audit your third-party scripts and remove any that are not providing significant value. Load the essential ones with defer.

4. Use a Web Worker:

  • For computationally expensive JavaScript, you can move it off the main thread and into a Web Worker, which runs in the background and doesn't block user interaction.

How to Improve Cumulative Layout Shift (CLS)

CLS is caused by elements on the page moving around after they have been initially rendered. This is a common annoyance, especially on mobile devices.

1. Always Include Size Attributes on Images and Videos:

  • The most common cause of CLS is images loading and pushing content down. Prevent this by always specifying the width and height attributes on your <img> and <video> tags. <img src="image.jpg" width="640" height="360" alt="...">

  • The browser will use these attributes to reserve the correct amount of space for the image before it has finished downloading.

2. Reserve Space for Ads and Embeds:

  • If you are loading ads, iframes, or other dynamic content, they can also cause layout shifts. Determine the size of the ad slot or embed and use CSS to reserve that space beforehand.

3. Avoid Inserting Content Above Existing Content:

  • Never dynamically inject content like banners or forms above existing content, unless it is in response to a user interaction.

4. Use CSS Transform for Animations:

  • When animating elements, use the CSS transform property instead of properties like top, left, or margin, which can trigger layout changes. transform animations are handled by the browser's compositor thread and do not cause layout shifts. transform: translateX(100px); is better than margin-left: 100px; for animations.

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