Understand what lazy loading is, how it works under the hood, and how to implement it correctly
to speed up your pages, reduce bandwidth usage, and improve Core Web Vitals.
Large images are often the biggest performance problem on modern websites.
Even when optimized, loading every image at once can delay rendering, block critical resources,
and slow down the user experience. Lazy loading solves this by loading images only when needed —
usually when they are close to entering the viewport.
What Is Lazy Loading?
Lazy loading is a technique that defers the loading of non-critical resources at page load time.
Instead of downloading all images, videos, and iframes immediately,
the browser loads them progressively as the user scrolls down the page.
In practical terms, lazy loading:
Reduces initial page weight and HTTP requests
Improves perceived performance and time to interact
For content-heavy pages, like blogs and product listings, lazy loading is almost mandatory
if you want a fast and responsive experience.
Native Lazy Loading with HTML
The simplest way to implement lazy loading nowadays is to use the
loading="lazy" attribute directly in the HTML of each image.
Most modern browsers support this natively.
Basic Example
With a single attribute, the browser automatically decides when to load the image,
based on scrolling and viewport position. No JavaScript is required for this basic setup.
You should apply native lazy loading to all images that appear below the fold,
especially long content sections and image-heavy grids.
Using Lazy Loading with WebP and <picture>
If you are serving modern formats like WebP (which you should),
you can still use lazy loading normally — just add the attribute to the fallback <img> tag.
The browser will:
Load the WebP version when supported
Fallback to JPG when necessary
Apply lazy loading behavior regardless of format
Lazy Loading Background Images
Background images defined in CSS do not support native lazy loading.
To avoid loading heavy backgrounds too early, you can use JavaScript with
the Intersection Observer API.
This pattern delays background image loading until the user is close to the section,
reducing initial load time without breaking design.
SEO and Core Web Vitals Benefits
Proper lazy loading is highly aligned with Google's performance recommendations.
By reducing initial payload and deferring non-visible assets,
you can see measurable improvements in:
CLS (Cumulative Layout Shift) – when combined with fixed width/height
Overall page speed and user satisfaction
These metrics directly influence search ranking and Discover eligibility,
making lazy loading a strategic SEO decision, not just a technical tweak.
Common Mistakes to Avoid
Even though lazy loading is simple, there are a few traps that developers fall into:
Lazy loading the main hero image or LCP element
Lazy loading images that are already visible on first paint
Not defining width and height, causing layout shifts
Overcomplicating with heavy JS libraries when HTML is enough
A good rule is: do not lazy load anything that appears in the initial viewport.
Those assets should be loaded normally to avoid hurting perceived performance.
Final Recommendation
Lazy loading is one of the highest-impact, lowest-effort optimizations you can apply.
Start by adding loading="lazy" to all non-critical images,
combine it with WebP and responsive techniques, and use Intersection Observer
only when necessary for complex cases like background images.
When implemented correctly, lazy loading makes your site feel faster,
more efficient, and much more enjoyable for users on any device.
