Lazy Loading

Lazy loading is a technique utilized in software and web development. It refers to a specific programming or design approach that, once implemented, helps delay the loading of the entire webpage by displaying only parts that are needed. Also known as “on-demand loading,” this technique effectively delays unnecessary processing and saves resources.

Lazy Loading vs Eager Loading

Lazy loading and eager loading are resource management strategies that improve performance and efficiency. Compared to lazy loading, eager loading ensures all resources are immediately available, reducing delays during user interactions. The possible drawbacks of eager loading are the increased initial load times and resource usage.

Lazy Loading Process

Lazy loading works by marking some resources as non-essential and loading them only when they are required by the user. The technique involves several steps:

1. Recognizing Non-Essential Resources

Initially, on-demand loading starts by identifying resources that can be deferred without negatively impacting the user experience. Usually, these resources are large or non-critical assets like images, videos, or non-essential JavaScript or CSS.

2. Loading Essential Content First

After excluding non-critical resources, lazy loading focuses on loading only the necessary content required for the initial view, often referred to as above-the-fold content. This term refers to the part of the webpage or application visible to the user during the loading process.

3. Setting Up Visual Placeholders

To keep visual continuity and prevent layout shifts, lazy loading often involves using lightweight placeholders during the loading process of deferred resources. These placeholders, such as skeleton screens, blank spaces, or loading spinners/animations, keep the layout stable until the content is fully loaded for the user.

4. Triggering Resource Loading

Trigger mechanisms enable resources to be loaded when a specific event occurs. Common triggers include viewport entry and user interaction.

• With viewport entry, resources are loaded as they enter the visible area of the user's screen. This method is commonly used for content or images that appear while scrolling.
• In contrast, user interaction triggers resource loading by clicking a button, hovering over an element, or expanding a section. This makes it an excellent option for accordions or modal popups.

5. Detecting Trigger Events

Lazy loading is usually implemented using JavaScript or built-in browser elements, such as the loading="lazy" attribute for images and iframes. The Intersection Observer API, a native browser feature, detects when elements come into the viewport.

6. Rendering Deferred Content

Once the resource is triggered, the deferred content is fetched from the server and rendered in its designated place. This happens by typically using asynchronous requests such as AJAX or Fetch API to ensure smooth and efficient loading without interrupting the user experience.

7. Updating Content Dynamically

The newly loaded resource is seamlessly integrated into the webpage, replacing the placeholders with the fully loaded content. This assures the user experience is not disrupted and maintains visual continuity.

Lazy Loading Optimization

Once implemented, lazy loading is an effective website optimization method, improving performance and resource management. Several strategies can help optimize on-demand loading:

• Images: lazy-loading images is good when performance and bandwidth are priorities. Compared to text, images are larger and often non-essential for the initial view of the page. Deferring image loading significantly improves metrics like Largest Contentful Paint (LCP), leading to better user experience and SEO.
• Iframes: iframes are resource-intensive and can slow down the initial page load. Lazy loading iframes delays the loading of embedded external content, reducing the browser's initial processing load.
• Fonts: lazy loading, among other font optimization techniques, is an excellent method to reduce font load and improve overall performance. Fonts, especially custom web fonts, can be large and take time to download. On-demand loading ensures that only necessary fonts are loaded upfront, while secondary fonts are fetched only when required.
• CSS: deferring the loading of non-critical CSS allows the browser to focus on rendering essential styles for above-the-fold content. This reduces the initial render time, making the page appear faster to users.
• JavaScript: when properly implemented, lazy loading positively impacts JavaScript rendering. Lazy loading JavaScript involves deferring the loading and execution of non-critical JavaScript files until needed. This can be achieved using dynamic importing, async and defer attributes, code splitting, and conditional loading.
• Intersection observer API: efficiently detects when elements enter the viewport, enabling precise on-demand loading of resources like images and other content.
• Prioritize critical resources: combining lazy loading with techniques like preloading for above-the-fold content ensures that critical resources are prioritized. For external embeds such as maps or ads, the loading="lazy" attribute defers their loading until needed.

Benefits and Drawbacks of Lazy Loading

Lazy loading is considered a good practice, especially for web optimization. It increases performance, saves bandwidth, and improves user experience. However, it also has some drawbacks.