What Is Lazy Loading and When Should You Use It on Your Website?

/ Uncategorized / By

If your website feels sluggish on first load, chances are your browser is trying to download every image, video, and embedded iframe before showing anything useful to the visitor. Lazy loading is one of the simplest and most effective techniques to fix that. In this guide, we will explain what lazy loading is, how it works under the hood, and when you should (or should not) use it on your website. wp-rocket.me has a solid rundown on this.

What Is Lazy Loading?

Lazy loading is a web performance technique that delays the loading of non-critical resources until they are actually needed by the user. Instead of downloading every image, video, or iframe when the page first opens, the browser only fetches them when they are about to enter the viewport (the visible part of the screen).

The concept comes from general computer programming, where you defer the initialization of an object until the moment it is required. Applied to the web, it means:

  • Faster initial page load
  • Less bandwidth consumed by both server and user
  • Lower resource usage on mobile devices
  • Better Core Web Vitals scores
website loading speed

How Lazy Loading Works

When a browser parses your HTML, it normally starts downloading every asset it finds: images, background media, embedded videos, iframes, and so on. With lazy loading, the browser is told: “Don’t bother fetching this yet. Wait until the user scrolls close to it.”

There are two main ways to implement it:

  1. Native browser lazy loading using the loading HTML attribute (supported by all modern browsers in 2026).
  2. JavaScript-based lazy loading using the IntersectionObserver API or a library, useful for background images, custom video players, or older edge cases.

Native HTML Lazy Loading Examples

The easiest and most reliable method is the native loading attribute. Here are the three most common use cases:

1. Lazy loading an image

<img src="hero-photo.jpg" alt="Team meeting" loading="lazy" width="1200" height="800">

2. Lazy loading an iframe (YouTube, Maps, etc.)

<iframe src="https://www.youtube.com/embed/xxxx" loading="lazy" width="560" height="315" title="Product demo"></iframe>

3. Lazy loading a video poster

The <video> tag itself does not accept loading="lazy", but you can achieve the same effect with the preload attribute:

<video controls preload="none" poster="video-thumb.jpg">
  <source src="demo.mp4" type="video/mp4">
</video>

The preload="none" value tells the browser not to fetch the video file until the user hits play.

When Lazy Loading Helps Performance

Lazy loading is a big win in these scenarios:

  • Long articles or blog posts with many inline images
  • E-commerce category pages displaying dozens of product thumbnails
  • Pages with embedded third-party iframes (YouTube, Google Maps, social widgets)
  • Image galleries and portfolios where users may never scroll to the bottom
  • Mobile-first sites where every kilobyte of bandwidth matters
website loading speed

When Lazy Loading Can Hurt You

Lazy loading is not a magic switch. Applying it incorrectly can damage both user experience and SEO. Here is what to watch out for:

1. Never lazy load above-the-fold content

Your Largest Contentful Paint (LCP) element (usually the hero image or main product photo) should load eagerly. Adding loading="lazy" to it will delay the paint and hurt your Core Web Vitals score.

<!-- Correct: hero image loads immediately -->
<img src="hero.jpg" alt="Homepage banner" fetchpriority="high" width="1600" height="900">

2. Watch out for layout shifts

If you don’t declare width and height on lazy-loaded images, the page will jump around as images load. This ruins the Cumulative Layout Shift (CLS) metric. There is a detailed walkthrough elsewhere.

3. SEO considerations

Googlebot renders pages and can index lazy-loaded content, but only if it is triggered correctly. Content hidden behind broken JavaScript lazy loaders may not be crawled. Native loading="lazy" is always safe for SEO.

4. Avoid over-lazy-loading

Lazy loading every asset on the page can actually make perceived performance worse. If the user scrolls fast, they will see a stream of empty placeholders.

Lazy Loading vs Eager Loading: Quick Comparison

Aspect Lazy Loading Eager Loading
Initial page weight Lower Higher
Time to first paint Faster Slower
Best for Below-the-fold assets Hero images, LCP element
Bandwidth saved Yes, if user does not scroll No
Risk of layout shift Yes, if dimensions missing Low

Common Lazy Loading Mistakes to Avoid

  1. Adding loading="lazy" to the LCP image. Always eager-load your hero.
  2. Forgetting width and height attributes. They reserve space and prevent CLS.
  3. Combining native lazy loading with a JavaScript library. They can conflict and cause double loading.
  4. Lazy loading CSS background images without a fallback. Search engines may not see them.
  5. Testing only on desktop. Always audit on real mobile devices and slow 3G.
  6. Applying lazy loading to icons or tiny sprites. The overhead is not worth it.
website loading speed

How to Test If Lazy Loading Is Working

  • Open Chrome DevTools, go to the Network tab, and reload the page. Images below the fold should not appear until you scroll.
  • Run Google PageSpeed Insights or Lighthouse and check the “Defer offscreen images” audit.
  • Use WebPageTest to compare the waterfall before and after enabling lazy loading.

Final Thoughts

Lazy loading is one of the highest return-on-effort optimizations you can apply to a modern website. In most cases, a single HTML attribute is enough to shave seconds off your load time and improve your Core Web Vitals. Just remember the golden rule: eager-load what the user sees first, lazy-load everything else.

Frequently Asked Questions

Is lazy loading good or bad?

It is good when used on below-the-fold assets like images, iframes, and videos. It becomes bad when applied to the hero image or when dimensions are not declared, because it can delay the LCP and cause layout shifts.

Is lazy loading bad for SEO?

No, native lazy loading via loading="lazy" is fully supported by Googlebot and does not hurt SEO. Poorly implemented JavaScript lazy loading, however, can prevent search engines from discovering content.

Which is better, lazy loading or eager loading?

Neither is universally better. Use eager loading for critical above-the-fold assets and lazy loading for everything the user has to scroll to reach.

Does lazy loading work on all browsers in 2026?

Yes. The native loading attribute is supported in every major browser, including Chrome, Edge, Firefox, Safari, and their mobile versions.

Can I lazy load videos?

The <video> tag does not accept loading="lazy", but setting preload="none" and using a poster image achieves the same effect. For embedded YouTube or Vimeo, use loading="lazy" on the iframe. A longer analysis is available for anyone who wants it.

Should I lazy load images on a landing page?

Yes for images below the fold. No for the hero image or any visual that appears in the first viewport, as it would delay your LCP metric.