Google Warns Against Default Lazy Loading Due to Largest Contentful Paint Performance Impact

Google's development team has identified how default lazy loading of images can significantly delay Largest Contentful Paint (LCP) metrics, potentially impacting core web vitals and search rankings. This insight emerged during a recent episode of Google's Search Off the Record podcast featuring Martin Splitt and John Mueller.

Understanding the Performance Impact

Martin Splitt highlighted a real-world example from developers.google.com where the content management system automatically applied lazy loading to all images. This default setting created unexpected performance issues, particularly for above-the-fold content.

"If you are using lazy loading on an image that is immediately visible, that is most likely going to have an impact on your largest contentful paint. It's almost guaranteed," Splitt emphasized.

The challenge occurs because lazy loading fundamentally changes how browsers prioritize image loading. When a hero image is lazy-loaded, the browser treats it as lower priority, allowing other resources to load first. Implementing proper edge caching strategies alongside optimized image loading can significantly impact LCP metrics, especially on slower networks or devices with limited processing power.

Technical Implementation Considerations

The rise of native lazy loading support in modern browsers has simplified implementation, with platforms like WordPress adopting it by default. However, some older or custom lazy-loading libraries pose additional risks for SEO. These libraries may hide image URLs in nonstandard attributes, potentially preventing Google from indexing the images properly.

"We've seen multiple lazy loading libraries that use some sort of data-source attribute rather than the source attribute… If it's not in the source attribute, we won't pick it up if it's in some custom attribute," Splitt noted.

Optimizing Implementation

To maximize website performance while maintaining SEO benefits, developers should implement the following strategies:

1. Maintain hero and above-the-fold images with eager-loading and defined width/height attributes
2. Apply native loading="lazy" exclusively to below-the-fold images and iframes
3. Verify implementation through Search Console's URL Inspection tool
4. Ensure lazy-loading libraries expose real URLs in standard attributes

Improving overall site search performance through optimized image loading remains crucial for both user experience and search engine visibility.

For additional technical guidance on implementing lazy loading correctly, developers can reference Google's Web.dev documentation on lazy loading best practices.

This development underscores the ongoing challenge of balancing performance optimization with user experience and search engine visibility in modern web development.