What is Lazy Loading and Preloading?

As websites become more complex and visually rich, optimizing performance has become a top priority for developers. Two critical techniques in web performance optimization are lazy loading and preloading.

What is Lazy Loading?

Lazy loading is a technique where certain assets, such as images, videos, or scripts, are loaded only when they are needed—typically when they come into the user's viewport. Instead of loading all resources upfront, lazy loading prioritizes essential content while deferring non-essential content.

How Lazy Loading Works

Lazy loading uses JavaScript to determine when an element is visible in the viewport. Once detected, the resource is fetched and displayed. Images may use a placeholder until the user scrolls to their position.

Benefits of Lazy Loading

• Faster Initial Load Time: Reduces data downloaded during initial page load
• Improved Performance: Frees up browser resources
• Better User Experience: Users can interact with the site sooner
• SEO Boost: Faster page loads improve search rankings

Common Use Cases

• Images on long-scrolling pages
• Embedded videos or iframes
• Third-party scripts and widgets
• E-commerce or content-rich websites

What is Preloading?

Preloading is a technique used to load specific resources in advance so they are readily available when needed. Unlike lazy loading, preloading fetches resources earlier in the page lifecycle.

How Preloading Works

Preloading tells the browser to prioritize certain resources using the <link rel="preload"> tag. This ensures critical assets like fonts, above-the-fold images, and CSS are available immediately.

Benefits of Preloading

• Faster rendering of critical content
• Smoother user experience
• Prevents layout shifts from late-loading fonts

Key Differences

• Lazy Loading: Defers loading until needed
• Preloading: Loads resources early before they're needed

Combining Both Techniques

For optimal performance, use preloading for above-the-fold critical resources and lazy loading for everything below the fold or off-screen.