The portion of images and video in the typical payload of a website can be significant. Unfortunately, project stakeholders may be unwilling to cut any media resources from their existing applications. Such impasses are frustrating, especially when all parties involved want to improve site performance, but can't agree on how to get there. Fortunately, lazy loading is a solution that lowers initial page payload and load time, but doesn't skimp on content.
What is lazy loading?
Lazy loading is a technique that defers loading of non-critical resources at page load time. Instead, these non-critical resources are loaded at the moment of need. Where images are concerned, "non-critical" is often synonymous with "off-screen". If you've used Lighthouse and examined some opportunities for improvement, you may have seen some guidance in this realm in the form of the Defer offscreen images audit:
You've probably already seen lazy loading in action, and it goes something like this:
- You arrive at a page, and begin to scroll as you read content.
- At some point, you scroll a placeholder image into the viewport.
- The placeholder image is suddenly replaced by the final image.
An example of image lazy loading can be found on the popular publishing platform Medium, which loads lightweight placeholder images at page load, and replaces them with lazily-loaded images as they're scrolled into the viewport.
If you're unfamiliar with lazy loading, you might be wondering just how useful the technique is, and what its benefits are. Read on to find out!
Why lazy-load images or video instead of just loading them?
Because it's possible you're loading stuff the user may never see. This is problematic for a couple reasons:
- It wastes data. On unmetered connections, this isn't the worst thing that could happen (although you could be using that precious bandwidth for downloading other resources that are indeed going to be seen by the user). On limited data plans, however, loading stuff the user never sees could effectively be a waste of their money.
- It wastes processing time, battery, and other system resources. After a media resource is downloaded, the browser must decode it and render its content in the viewport.
Lazy loading images and video reduces initial page load time, initial page weight, and system resource usage, all of which have positive impacts on performance.
Implementing lazy loading
There are a number of ways to implement lazy loading. Your choice of solution must take into account the browsers you support, and also what you are trying to lazy-load.
Modern browsers implement browser-level lazy loading,
which can be enabled using the
loading attribute on images and iframes.
To provide compatibility with older browsers
or to perform lazy loading on elements without built-in lazy loading
There are also a number of existing libraries to help you to do this.
See the posts on this site for full details of all of these approaches:
Also, we have compiled a list of potential issues with lazy loading, and things to watch out for in your implementation.
Used with care, lazy loading images and video can seriously lower the initial load time and page payloads on your site, including Core Web Vitals. Users won't incur unnecessary network activity—including network contention on slower connections—and processing costs of media resources they may never see, but they can still view those resources if they want.
As far as performance improvement techniques go, lazy loading is reasonably uncontroversial. If you have a lot of inline imagery in your site, it's a perfectly fine way to cut down on unnecessary downloads. Your site's users and project stakeholders will appreciate it!