Lazy loading (English "delayed loading") is a web optimization technique that ensures that images, videos or other content are loaded only when the user actually needs them - i.e. only when it approaches the visible part of the screen (viewport). As opposed to the classic loading, where all the content of the page is downloaded as soon as it is opened, lazy loading saves data transfer and speeds up page loading.
If a site contains a lot of images or videos, loading them immediately can slow down the site speed significantly. Lazy loading solves this problem by loading content sequentially.
The main advantages of lazy loading:
- Faster page loading - the page is displayed to the user earlier because not all elements are loaded at once. A slow website has major impact on your conversions.
- Better user experience (UX) - Visitors don't have to wait for elements to download that they may never scroll to.
- Lower server load - less data is transferred at once, which improves performance.
- Positive impact on SEO - Faster website loading is a proven ranking factor in search engines.
How does lazy loading work in practice?
- No lazy loading:
All images and videos on the page are downloaded as soon as they load, regardless of whether the user sees them. - With lazy loading:
Only elements that are visible on the screen are loaded. Others are loaded only when the user scrolls to them.
How to implement lazy loading?
- HTML attribute
loading="lazy"- modern and simple way. Just add this attribute to your images:<img src="obrazek.jpg" alt="Picture description" loading="lazy"> - JavaScript libraries - for example lazysizesthat allow more advanced settings and work with older browsers.
- CMS and plugins - most content management systems (e.g. WordPress) already support lazy loading natively or via plugins.
When to use lazy loading and when not?
Lazy loading is great for content that isn't immediately visible - typically images at the bottom of the page, galleries, product catalogues or videos. However, it should not be used on elements that are at the top of the page and crucial to the first impression, because their delay would be distracting.
Summary
Lazy loading is a simple and effective technique that helps to speed up a website, improve UX and boost SEO. It is easy to implement - just add an attribute in modern HTML loading="lazy". For sites with a lot of multimedia content, this optimization is almost a must.
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