What is lazyloading?

Lazy loading is a technique where a website or app only loads the parts of a page that are needed right now, and postpones loading the rest until the user actually needs it. Think of it like turning on a light only when you walk into a room, instead of keeping every light on all the time.

Let's break it down

  • Initial load: When you first open a page, the browser loads the essential HTML, CSS, and the most important images or scripts.
  • Placeholder: Anything that isn’t needed immediately is replaced with a tiny placeholder (like a blank space or a low‑resolution thumbnail).
  • Trigger: As you scroll or interact, the browser detects that a hidden element is about to become visible.
  • Load on demand: At that moment, the real content (image, video, component, etc.) is fetched and displayed.

Why does it matter?

  • Faster start‑up: The page appears quicker because there’s less data to download at first.
  • Less data usage: Users on slow connections or limited data plans only download what they actually view.
  • Better performance: Reduces memory and CPU load, which helps the site run smoother on phones and older computers.

Where is it used?

  • Images: Photo galleries, news sites, and e‑commerce product lists often lazy‑load pictures.
  • Videos: Streaming platforms load video players only when you scroll near them.
  • Infinite scroll: Social feeds (Facebook, Instagram) load more posts as you keep scrolling.
  • Heavy components: Large JavaScript widgets, maps, or ads that aren’t needed right away.

Good things about it

  • Improves page load speed and perceived performance.
  • Saves bandwidth for both the site and the user.
  • Can boost SEO because search engines favor faster pages.
  • Enhances user experience on mobile devices with limited resources.

Not-so-good things

  • If not implemented correctly, users may see a flash of empty space or a “pop‑in” effect.
  • Search engine crawlers might miss lazy‑loaded content if they can’t execute JavaScript.
  • Adds extra code complexity, which can introduce bugs if the detection logic fails.
  • May cause layout shifts, affecting the page’s visual stability (a factor in Core Web Vitals).