Is lazy loading really essential to optimize your initial page weight and boost Core Web Vitals?

Why does Google insist so much on initial page weight?

A page's initial weight directly determines how quickly it becomes interactive for the user. The higher this weight, the longer the browser takes to download, parse, and display critical content. This is a determining factor for Core Web Vitals, particularly Largest Contentful Paint (LCP).

Lazy loading tackles this problem head-on: instead of loading 50 images when the page opens, you only load the ones visible on screen. The rest waits for the user to scroll. Result: fewer initial HTTP requests, less bandwidth consumed, faster Time to Interactive (TTI).

How does lazy loading actually work in practice?

The technique relies on the native HTML attribute loading="lazy" for images and iframes, or on JavaScript scripts that detect the viewport position. When an element approaches the visible area (typically with a safety margin), the browser triggers its loading.

Google explicitly recommends using the native attribute rather than complex JavaScript solutions. It's faster, better supported, and most importantly, Googlebot understands it perfectly. Custom scripts can block indexation if misconfigured.

Which content should you prioritize for lazy loading?

All heavy elements that are not above the fold: images at the bottom of the page, embedded videos, secondary carousels, third-party widgets. Conversely, never touch critical images (logo, hero image, first visual) — they must load immediately to avoid degrading LCP.

• Native lazy loading: use the loading="lazy" attribute on non-critical images and iframes
• Triggering threshold: the browser typically loads elements 1-2 screens before they become visible
• Positive SEO impact: reduction in initial weight, improvement of Core Web Vitals, better crawl budget
• Risk to avoid: never lazy-load images above the fold or those essential to main content
• Compatibility: the native attribute has been supported by all modern browsers since 2019

Is this recommendation consistent with what we observe in the field?

Absolutely. Audits I conduct consistently show that sites that properly lazy-load their images gain 20 to 40% on their LCP score. It's one of the most effective quick wins for improving Core Web Vitals without overhauling your entire architecture.

But — and this is a big but — I still see too many sites lazy-loading all their images, including header images. Catastrophic result: LCP explodes because the main image takes 2 seconds longer to appear. Google won't forgive this mistake.

What nuances doesn't Google mention here?

Martin Splitt talks about "heavy content" without specifying a threshold. Concretely, is a 30 KB image worth lazy-loading? Probably not. The gain will be negligible compared to the cost of the detection script. [To verify]: Google never gives precise figures on the minimum weight justifying lazy loading.

Another gray area: impact on image indexation. Googlebot is supposed to execute JavaScript and discover lazy-loaded images, but in practice, I've seen cases where certain images no longer appeared in Google Images after migrating to lazy loading. The cause? A misconfigured third-party script that blocked rendering on the bot side.

In what cases is lazy loading counterproductive?

On very short pages with few images (simple landing pages, minimalist product pages), lazy loading adds complexity for virtually no gain. Worse: if you lazy-load your product page's only image, you slow down its display for no reason.

Another problematic case: e-commerce sites with infinite scrolling product grids. If the lazy loading script is too aggressive (triggering threshold too close), users scrolling quickly see empty placeholders. This degrades UX and can increase bounce rate. You need to calibrate the preloading threshold carefully.

How to implement lazy loading without SEO risk?

Prioritize the native HTML attribute loading="lazy" on all <img> and <iframe> tags located below the first screen. It's the simplest method and best supported by Googlebot.

For critical images (hero, logo, first product image), use loading="eager" or don't include anything — the browser will load them immediately. If you're unsure what's "above the fold", test on multiple screen resolutions (mobile 375px, tablet 768px, desktop 1920px).

What mistakes must you absolutely avoid?

Never lazy-load images that contribute to Largest Contentful Paint. Use Lighthouse or PageSpeed Insights to identify this element — if it's an image, it must load with priority, not lazily.

Also avoid automatic WordPress plugins that lazy-load everything indiscriminately. I've seen sites lose 30% of organic traffic after activating a poorly configured plugin that blocked image indexation. Always test after deployment.

• Add loading="lazy" only to images and iframes below the fold line
• Keep loading="eager" (or no attribute) for critical above-the-fold visuals
• Verify in Search Console that Googlebot sees all your images after implementation
• Test your Core Web Vitals with PageSpeed Insights before/after to measure real gains
• On mobile, be especially careful: what's "below the fold" on desktop may be visible on small screens
• Disable lazy loading on pages with little content (< 2 screens in height)
• Monitor your rankings in Google Images for 2-3 weeks post-deployment