Images typically account for 50–70% of a webpage's total weight. On a page that weighs 3 MB, chances are 2 MB of that is images. This single fact is why image size is one of the highest-leverage things you can fix for both SEO and user experience.
How Google Uses Page Speed as a Ranking Signal
Since the Page Experience update in 2021, Google has officially incorporated Core Web Vitals into its ranking algorithm. Core Web Vitals are three metrics that measure real-world user experience:
- LCP (Largest Contentful Paint) — how fast the largest visible element loads. This is almost always an image.
- CLS (Cumulative Layout Shift) — how much page elements shift around unexpectedly. Images without dimensions cause layout shifts.
- INP (Interaction to Next Paint) — how responsive the page is to user interactions.
If your LCP element (usually a hero image or above-the-fold photo) is a 2 MB JPEG, Google is going to see a slow LCP. That's a direct ranking penalty.
The Direct Connection: File Size → Load Time → Rankings
Here's the simple chain of events that hurts your SEO:
- Large image file → more bytes the browser has to download
- More bytes → slower network load time
- Slower load time → worse LCP score
- Worse LCP → lower Core Web Vitals score
- Lower score → potential ranking disadvantage vs competitors
Studies by Google and Deloitte found that a 0.1 second improvement in load speed can improve conversion rates by 8%. The relationship between speed and business outcomes is well-established.
What "Large" Actually Means
There's no universal rule, but these are sensible thresholds to work from:
- Hero/banner images: aim for under 200 KB in WebP format
- Product/feature images: aim for under 100 KB
- Thumbnails and icons: should be under 30 KB
- Any image over 500 KB is almost certainly unoptimized
Image Inspector flags images over 500 KB as critical and 100–500 KB as warnings — thresholds based on real-world performance budgets.
The "Retina Tax" — Serving Images Bigger Than They Display
One of the most common and costly mistakes is serving images at their original resolution when they display much smaller. A photographer uploads a 4000×3000 JPEG from their camera. The website displays it at 800×600. The browser downloads the full 4000×3000 image anyway and throws away 93% of the pixels.
This "oversized image" problem is one of the most impactful things to fix. If a 3 MB image displays at 25% of its natural size, you can safely resize it and save roughly 75% of the file size — without any visible quality loss.
Image Size and Mobile SEO
Google uses mobile-first indexing, meaning it primarily crawls and ranks the mobile version of your site. Mobile connections are often slower and more bandwidth-constrained. An image that loads acceptably on desktop fiber can take 4–5 seconds on a 4G mobile connection.
The fix isn't just to compress images — it's to serve appropriately sized images per device using the srcset attribute. Image Inspector flags images that are missing srcset, which is a warning that mobile users may be downloading unnecessarily large files.
How to Find and Fix Oversized Images
The fastest way to identify image size issues is to use Image Inspector. Install it from the Chrome Web Store, navigate to any page, and click Scan. Within a few seconds you'll see:
- Every image on the page with its actual file size
- A comparison of natural vs displayed dimensions
- The potential savings if each image were properly optimized
- Specific recommendations (convert to WebP, resize, add lazy loading)
For a typical e-commerce or content-heavy site, it's common to find 2–5 MB of potential savings on a single page. That's the difference between a good LCP score and a failing one.
Key Takeaways
- Images are the largest contributor to page weight on most websites
- LCP — Google's key speed metric — is usually an image
- Serving images larger than they display wastes bandwidth and hurts LCP
- Mobile-first indexing means slow mobile image loads directly affect rankings
- Tools like Image Inspector make it easy to find and quantify these issues in seconds