What's the impact of using lazy loading on LCP and CLS metrics?

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What's the impact of using lazy loading on LCP and CLS metrics?

Asked on Dec 30, 2025

Answer

Lazy loading can significantly improve the Largest Contentful Paint (LCP) by deferring the loading of offscreen images, allowing the browser to prioritize critical resources. However, improper implementation can lead to Cumulative Layout Shift (CLS) if dimensions are not specified, causing layout changes when images load.

Example Concept: Lazy loading defers the loading of non-critical images and iframes until they are about to enter the viewport, reducing initial load times and improving LCP. To avoid negative impacts on CLS, always specify width and height attributes or use CSS to reserve space for lazy-loaded elements.

Additional Comment:

Lazy loading is particularly beneficial for pages with many images below the fold.
Ensure that critical images, such as hero images, are not lazy-loaded to prevent delays in LCP.
Use the "loading" attribute set to "lazy" on img and iframe elements for native browser support.
Monitor CLS by reserving space for images using CSS or by setting explicit dimensions.

✅ Answered with Core Web Vitals best practices.

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