Why are your images slowing down the mobile speed of your site?

Why does Google place so much emphasis on the weight of mobile images?

The figure presented by Google is staggering: 65% of the data transferred on mobile comes from images. On a smartphone with an unstable 4G connection or in a poorly covered area, every kilobyte counts. A visitor who waits more than 3 seconds will abandon, and Google knows this all too well.

This massive proportion can be explained by the evolution of modern websites. HD images, high-resolution produced visuals, full-width backgrounds: all of this inflates page weight. Meanwhile, mobile connections remain inconsistent based on geographical regions and operators.

What does “lossless compression” really mean in this context?

Google talks about lossless compression, but let’s be honest: in practice, controlled lossy compression is often more relevant. WebP, AVIF, or even a JPEG compressed to 80-85% quality offer an unbeatable quality/weight ratio. Pure lossless compression remains marginal for very specific cases like logos or technical diagrams.

The mobile size adaptation mentioned by Google concerns responsive images: serving an 800px wide image to a 375px screen is pure waste. The srcset attribute and modern formats allow for serving exactly what the device needs, no more.

What is the direct link between this and SEO and Core Web Vitals?

Loading time directly affects LCP (Largest Contentful Paint), one of the three Core Web Vitals metrics. If your largest visible element is a 2 MB image, your LCP will be catastrophic. Google has clearly integrated this into its ranking criteria since the Page Experience update.

Beyond ranking, it’s the bounce rate and time on site that suffer. A frustrated mobile user due to slowness will never see your content, no matter how high quality it is. Thus, image optimization becomes a non-negotiable prerequisite for any serious SEO strategy.

• 65% of mobile data comes from images according to Google
• Lossless compression mentioned, but controlled lossy compression is often more effective in practice
• Responsive adaptation via srcset and modern formats (WebP, AVIF) is essential
• Direct impact on LCP and thus on the Core Web Vitals ranking
• Inconsistent mobile connections: optimization is critical for real accessibility

Does this statement really reflect on-the-ground observations?

The figure of 65% aligns with what we observe on most e-commerce, media, or corporate sites. Regular PageSpeed audits consistently show that unoptimized images are the primary factor for slowdowns. Google isn't pulling this number out of thin air.

However, the wording remains deliberately vague regarding the recommended formats. Google does not explicitly say “use WebP or AVIF,” even though these are the formats that yield the most significant gains. This caution is likely due to compatibility issues still present in some old browsers. [To verify] in your analytics: what percentage of your traffic still uses browsers incompatible with WebP?

What nuances should we bring to this recommendation?

Not all sites face this issue equally. A text-based blog with some illustrations will never be as impacted as a photo portfolio site or a fashion store. Adapting the optimization effort to your business reality is essential.

Another point: pure lossless compression can be counterproductive. On a site with thousands of product images, aiming for zero visual loss is expensive in infrastructure and processing time, yielding a gain that is imperceptible to the naked eye. The goal is not technical perfection; it’s the balance between perceived quality and real performance.

In what cases can this rule be moderated?

If your target audience is exclusively B2B on desktop with fiber optic, the urgency is not the same as for a mobile-first public site. But be careful: Google has been indexing mobile-first since 2019, so even if your visitors are on desktop, Googlebot crawls you on a smartphone.

Sites with critical images for conversion (real estate, art, high-end fashion) must find a subtle compromise between quality and speed. In these cases, aggressive lazy loading, a fast CDN, and adaptive formats become even more strategic. Never sacrifice perceived quality at the altar of raw performance if it differentiates you from competitors.

What concrete actions should you take immediately on your site?

The first action: audit the actual weight of your images using PageSpeed Insights or GTmetrix. Identify the 10-15 heaviest images that appear on your strategic pages. These are the ones that are dragging down your LCP and mobile conversion rate.

Next, implement a strategy for adaptive formats. Serve WebP with JPEG fallback for older browsers. If you are on WordPress, plugins like ShortPixel or Imagify automate this process. For custom sites, correctly implement srcset and sizes, not just defaults.

What critical mistakes should you absolutely avoid?

Classic mistake: resizing images in CSS rather than at the source. Displaying a 3000px wide image with a max-width: 600px does not reduce the weight downloaded, just the display. The browser still loads the entire 3000px.

Another trap: compressing too much. A pixelated image or one with visible JPEG artifacts damages your brand image and your conversion rate. Always visually test on multiple screens before deploying aggressive compression. The human eye remains the best judge.

How can you check if your optimizations are effective?

Use PageSpeed Insights in mobile mode and track the evolution of your LCP score for at least 4 weeks. Fluctuations can be significant depending on Google updates. A good mobile LCP is under 2.5 seconds, ideally under 2 seconds.

Also compare your mobile bounce rate before/after optimization. If your images are truly the bottleneck, you should see a measurable improvement in mobile engagement rates within 2-3 weeks following deployment. Analytics does not lie.

• Audit the 10-15 heaviest images on your strategic pages
• Implement WebP with JPEG fallback for maximum compatibility
• Configure srcset and sizes to serve the correct resolution to the right screen
• Enable lazy loading on all below-the-fold images
• Visually test quality after compression on a real mobile device
• Monitor mobile LCP and bounce rate for at least 4 weeks