How to Use LazyLoad for Images (And When Not To)

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Optimizing the performance of web pages is a critical focus for developers, and one effective optimization technique is image lazy loading. By deferring the loading of images until they’re needed—typically when they scroll into the user’s viewport—you can enhance page speed, reduce initial load time, and improve overall user experience. But while lazy loading presents clear advantages, it’s crucial to know when and how to use it properly to avoid unintended performance or accessibility issues.

TL;DR

Lazy loading is a technique that delays loading images until they’re needed, which helps speed up initial page loads and reduces bandwidth consumption. It is especially helpful for pages with many images or long scrollable content. However, it shouldn’t be applied to above-the-fold or critical content, as it can hinder user experience and affect SEO. Use it strategically and always monitor user interaction and performance outcomes.

What Is Lazy Loading?

Lazy loading refers to the practice of delaying the loading of non-critical resources at page load time. When applied to images, lazy loading means these images will only be fetched and rendered when they come close to entering the viewport or when the user is likely to need them.

This approach helps minimize the number of resources being requested upfront, and as a result, it improves performance, especially on slower networks or less powerful devices.

Why Use Lazy Loading?

There are several compelling reasons to use lazy loading for images, including:

  • Improved Page Speed: Fewer HTTP requests during page load speed up rendering.
  • Reduced Bandwidth Usage: Only images that the user needs are loaded, which can save data.
  • Better User Experience: Faster initial loads lead to lower bounce rates and higher engagement.
  • Enhanced Performance on Mobile: Mobile networks benefit greatly from deferred image loading.

How Lazy Loading Works

Lazy loading typically relies on either native browser support or JavaScript-based libraries that observe scrolling behavior and replace placeholder images with actual content when necessary. The loading process can be triggered either by events (like scroll) or by APIs like Intersection Observer.

There are two primary ways to implement lazy loading:

1. Native HTML Lazy Loading (Recommended)

The simplest and most compatible way to lazy load images today is by using the loading="lazy" attribute on your <img> elements:

<img src="image.jpg" loading="lazy" alt="Description">

This method is supported in most modern browsers and doesn’t require any JavaScript.

2. JavaScript Libraries

For browsers not supporting native lazy loading or for more advanced control, JavaScript libraries such as Lozad.js, LazyLoad.js, or lazysizes can be employed. These libraries often provide additional functionalities such as animations, placeholder effects, and compatibility fallbacks.

Here’s a basic usage example using the popular lazysizes library:

<img data-src="image.jpg" class="lazyload" alt="Description">

When to Use Lazy Loading

While lazy loading seems like an all-around advantage, strategic usage is important. Below are some scenarios where using lazy loading is appropriate:

  • Long, Scrollable Pages: When pages contain a large number of images, especially galleries or blog feeds.
  • Image-Heavy Content: E-commerce sites, portfolios, or photography blogs that use many media resources.
  • Below-the-Fold Images: For content not immediately visible to the user when the page loads.
  • Mobile-Optimized Sites: Mobile users often have limited data and slower connections.

When Not to Use Lazy Loading

There are cases where opting for lazy loading may cause more harm than good. Poor usage can negatively impact accessibility, SEO, or perceived performance. Avoid lazy loading in the following scenarios:

  • Above-the-Fold Content: Critical visual elements should load immediately to avoid layout shifts and delays.
  • Hero Images and Logos: Branding and key visuals queued under lazy loading may confuse or distract users.
  • SEO-Critical Images: If search engine optimization depends on specific images, lazy loading may prevent them from being indexed, especially on JavaScript-dependent implementations.
  • Users with JavaScript Disabled: If your lazy loading approach depends on JS, these images may never load at all.

Search Engine Optimization (SEO) Considerations

With the growing importance of performance metrics like Largest Contentful Paint (LCP) and Core Web Vitals, lazy loading can indirectly affect SEO—for better or worse.

If Google’s bots cannot easily index your images because they’re not present in the initial HTML or depend on JS, your images might not rank. However, Google’s bots do support native lazy loading to an extent, and modern crawlers execute JavaScript. To be safe:

  • Use native lazy loading whenever possible.
  • Ensure important images are not lazy-loaded.
  • Include unique and descriptive alt attributes for all images.

Accessibility and Usability Tips

Beyond performance and SEO, accessibility must not be sidelined. Improper implementation of lazy loading could lead to:

  • Images not loading for screen readers if they’re never inserted into the DOM.
  • Keyboard navigation skipping images that haven’t loaded yet.

Always test your implementation with accessibility tools and diverse devices. Use semantic HTML and provide descriptive alt text to all images, ensuring screen readers interpret your content correctly.

Best Practices for Implementing Lazy Loading

Here are technical and strategic best practices you should follow for optimum results:

  • Use native support (loading="lazy") when possible—it’s simple and efficient.
  • Ensure the first-screen images are eagerly loaded (without lazy loading tag).
  • Provide meaningful alt attributes for all images.
  • Use low-quality placeholders (LQIP) to indicate pending loading areas.
  • Monitor real-world performance using tools like Web.dev or PageSpeed Insights.
  • Combine lazy loading with responsive images for the best results on varying screen sizes.

Tools for Lazy Loading

Here are some trusted libraries and tools used for implementing image lazy loading:

  • lazysizes – High-performance lazy loader with various plugins and auto-detection.
  • Lozad.js – Lightweight and performant.
  • vanilla-lazyload – Easy to use and SEO-friendly.

Conclusion

Lazy loading is a powerful and easy-to-implement optimization technique that reduces bandwidth usage, boosts page loading speeds, and improves user experience. It’s especially beneficial for media-heavy pages and mobile-first websites. However, its effectiveness depends heavily on careful and strategic application.

By understanding when and how to apply lazy loading, web developers can ensure their pages are both fast and functional—without sacrificing user experience or search engine visibility. When in doubt, test across browsers, devices, and user scenarios to achieve the best possible outcome.