How Image Lazy Loading Improves Core Web Vitals

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This is the last installment of a three-part series on Core Web Vitals and image optimization. The last two parts are:

This blog series examines important ways to improve Core Web Vitals (CWV) — and SEO — by optimizing image assets. In the last article, we discussed how to improve Largest Contentful Paint with proper image compression, resizing, and next-gen image conversion. In addition to those three techniques, you will likely see Defer Offscreen Images, also called lazy loading, as one of the improvement opportunities in your PageSpeed Insights analysis. In this article, we’ll focus on this technique and talk about what it is, why it’s important, and how to implement it.

Defer offscreen images

What Is Lazy Loading?

Imagine an e-commerce page with 100 high-resolution photos of shoes. If a browser loads all the photos at once, some towards the bottom of the page might appear before you can scroll to them, while photos in your viewport might not. This can create a bad user experience.

Lazy loading is the technique that defers offscreen images from loading until the visitor scrolls and the images enter the viewport. It allows fewer images to load at any given time and speeds up the load time for each image, which can improve Largest Contentful Paint.

Lazy loading can improve First Input Delay (FID) as well. FID is the time it takes for a page to respond to the user's first interaction, such as a click. By strategically delaying large components like images from loading, the website can process JavaScript and respond to interactions faster.

How to Implement Lazy Loading

How you implement lazy loading depends on your code base and the browsers you support. In addition to loading=“lazy”, a Chrome-supported HTML attribute, developers can use JavaScript and/or Intersection Observer API requests. The following chart from caniuse.com shows which browsers support the HTML lazy loading attribute:

lazy loading

However, given how fast browsers are changing and their unique capabilities, developers increasingly consider using the lazysizes.js JavaScript as a best practice. In the following video, our partners at Cantilever demonstrate and compare the loading=“lazy” and lazysizes.js implementations.

Best Practice: lazysizes.js + imgix.js

Using lazysizes.js together with imgix.js is a great way to combine the benefits of imgix image optimization and lazy loading. Here’s an example of how it’s done as a meta property:

<head>
  <meta property="ix:srcAttribute" content="data-src">
  <meta property="ix:srcsetAttribute" content="data-srcset">
  <meta property="ix:sizesAttribute" content="data-sizes">
</head>

And here’s an example of how it’s done in JavaScript:

imgix.config.srcAttribute = 'data-src';
imgix.config.srcsetAttribute = 'data-srcset';
imgix.config.sizesAttribute = 'data-sizes';

Use Placeholders with Lazy Loading

The risk of lazy loading is that it could increase the page layout instability, leading to a worse Cumulative Layout Shift score. For example, when a visitor scrolls down quickly and starts to read an image caption, the image can suddenly appear, pushing the caption out of view.

The way to mitigate this issue is to use low-quality placeholders or BlurHash, which will occupy a frame until it’s appropriate to load a fully detailed version.

Resources

This blog concludes our series on Core Web Vitals. We hope you find it helpful in your efforts to improve website performance. If you missed the earlier articles in the series, you could find them here:

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