What is lazyload?

Lazyload (or lazy loading) is a technique that delays loading of images, videos, or other heavy resources until they are actually needed-usually when they become visible on the screen as the user scrolls. Instead of downloading everything when the page first opens, the browser only fetches what the user can see right away.

Let's break it down

  • Initial page load: The browser loads the HTML, CSS, and a small set of essential assets.
  • Placeholder: For each lazy‑loaded item, a lightweight placeholder (often a blank space or low‑resolution thumbnail) is placed in the markup.
  • Trigger: When the user scrolls and the placeholder is about to enter the viewport, a script detects this event.
  • Load: The script swaps the placeholder with the real image/video URL, prompting the browser to download the full resource.
  • Display: Once downloaded, the real content appears seamlessly.

Why does it matter?

  • Speed: Faster initial page load because fewer bytes are transferred up front.
  • Performance: Reduces memory and CPU usage, especially on mobile devices.
  • User experience: Users see content quicker, and scrolling feels smoother.
  • Bandwidth savings: Only the resources the user actually views are downloaded, saving data for users on limited plans.

Where is it used?

  • Image galleries and blogs: Large collections of photos that are below the fold.
  • Infinite scroll feeds: Social media timelines, news feeds, or product listings.
  • Video platforms: Loading video thumbnails first, then the video file when the user clicks or scrolls near it.
  • Web apps: Any page with heavy assets that aren’t needed immediately, such as dashboards with many charts.

Good things about it

  • Improves page load time and Core Web Vitals scores.
  • Lowers server load and bandwidth costs.
  • Enhances SEO when implemented correctly (search engines can still index lazy‑loaded images).
  • Simple to add with native HTML (loading="lazy" attribute) or small JavaScript libraries.

Not-so-good things

  • If not implemented carefully, images may appear late, causing a “pop‑in” effect that can be jarring.
  • Some older browsers don’t support native lazy loading, requiring polyfills.
  • Search engine crawlers might miss content if lazy loading blocks indexing.
  • Complex layouts may need extra code to calculate when an element is truly in view, increasing development effort.