Largest Contentful Paint (LCP) is a critical metric used by Google to measure how quickly the main content of a webpage is visually loaded for users. Recently, Google’s Martin Splitt explained on the “Search Off the Record” podcast why default lazy loading can delay LCP, the impact it has on user experience and SEO, and how developers can verify and fix these issues. This blog post explores why lazy loading sometimes hurts performance, especially on above-the-fold images, and actionable best practices to avoid delays.

What Is Largest Contentful Paint and Why Does It Matter?

Largest Contentful Paint measures when the largest block of visible content—often the hero image or prominent text—finishes rendering in the viewport. Google recommends an LCP time of under 2.5 seconds for a good user experience. If LCP is slow, visitors may feel the page is sluggish or incomplete, impacting engagement and even search rankings indirectly.

Images play a pivotal role in LCP since the largest visual element is commonly an image, such as a banner or hero shot. Therefore, optimizing how and when these images load is essential to achieve fast LCP.

How Lazy Loading Can Delay Largest Contentful Paint

Lazy loading is a technique that defers loading images or other resources until they are needed, typically when they scroll into view. This saves bandwidth and speeds up initial page load for content below the fold. However, applying lazy loading indiscriminately, especially to above-the-fold images that are critical to LCP, can backfire.

Martin Splitt illustrated this with the example of developers.google.com, which by default uses lazy loading on all images—including hero images above the fold. He said, “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 like almost guaranteed.”

Here’s why it happens:

• The browser’s preload scanner usually detects above-the-fold images early and loads them with high priority.
• When loading="lazy" is added to such images, they are deprioritized.
• The browser waits for other critical work such as scripts and styles to complete before fetching these lazy images.
• This causes the hero image to load later, pushing back the moment the largest visual content paints.
• On slow networks or devices with limited CPU power, the delay is even more noticeable.
• If the width and height attributes aren’t set, the late image load can cause layout shifts, which feel jarring to users.

The net effect: lazy loading essential visible content delays LCP and harms both user experience and potentially SEO.

SEO Risks from Lazily Loaded Images with Improper Markup

Besides delaying LCP, some lazy-loading libraries use experimental approaches that hide image URLs in non-standard attributes like data-src instead of the usual src or srcset. Google’s indexing relies on URLs being present in standard attributes in the rendered HTML.

Splitt warned: “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.”

Therefore, these implementations can cause images not to be indexed properly, which can affect the site’s visibility in image search or broader SEO.

How to Verify and Fix LCP Delays Due to Lazy Loading

Google recommends these best practices to prevent lazy loading from delaying LCP:

1. Keep above-the-fold images eager: Do not apply lazy loading to hero images or any visuals immediately visible when the page loads. These images should load with high priority.
2. Set explicit width and height: Always specify width and height attributes for images to prevent layout shifts as images load.
3. Use native loading="lazy" for below-the-fold content: For images or iframes that appear later in the scroll, lazy loading is beneficial to reduce bandwidth and improve overall page speed.
4. Check rendered HTML: Use Google Search Console’s URL Inspection tool to confirm that above-the-fold images have their URLs in standard src or srcset attributes in the rendered HTML rather than custom or JavaScript-based lazy loading.
5. Avoid heavy JavaScript libraries for lazy loading: Favor the native browser loading attribute over third-party lazy loading scripts that may interfere with indexing and image visibility.

Ranking Impact of Lazy Loading on LCP

While Google’s Martin Splitt framed the impact on rankings as generally modest, LCP is one of the Core Web Vitals metrics contributing to page experience signals. Even a small delay in LCP can affect user perception of site speed and usability, which correlates with search engagement metrics.

Splitt remarked: “Core Web Vitals contribute to ranking, but it’s a tiny minute factor in most cases.” Nonetheless, striving for optimal LCP by avoiding lazy loading on critical images is a best practice to ensure fast-loading, user-friendly pages.

Summary: Use Lazy Loading Wisely to Avoid LCP Delay

Lazy loading is valuable for optimizing web performance when applied selectively. The key takeaway from Google’s insights is to treat lazy loading as an opt-in feature for noncritical, below-the-fold content. Eagerly load hero images and other immediately visible content to avoid delaying the Largest Contentful Paint metric.

By verifying implementations using Search Console and rendering tools, and aligning with native browser capabilities, developers can maintain fast LCP, smooth user experiences, and SEO benefits in their search marketing efforts.

This blog has explained why lazy loading can delay Largest Contentful Paint (LCP) according to Google expert Martin Splitt, highlighted SEO risks with incorrect lazy loading, and shared best practices to prevent performance degradation.

For webmasters and SEO professionals aiming to balance performance and speed, understanding this nuance is crucial in optimizing websites for both users and search engines.

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