Why image compression is essential to avoid a slow website
The technical detail that costs the most in performance, often without anyone noticing
Does your site feel fast to you? That’s normal: your browser has already cached everything. For a visitor discovering your page for the first time, on 4G, the experience is often quite different. In the vast majority of cases, uncompressed images explain most of that slowness.
Image weight you can’t see, but you can feel while loading
A high-resolution photo may look crisp and professional on screen, but every extra megabyte adds to the loading time, especially on mobile. This problem affects almost every website: images get added, displayed, and then forgotten, without ever being properly optimised.
Outdated formats and oversized dimensions
Two habits explain most of the slowdown. First, the near-systematic use of JPEG and PNG formats, when newer formats such as WebP or AVIF offer significantly better compression for equivalent visual quality. Second, uploading images far larger than their actual display size: a 4000-pixel-wide image displayed at 800 pixels forces every visitor to download three times more data than necessary, with no visual benefit whatsoever.
A direct impact on Google ranking and on visitors
This extra weight slows down a key Google Core Web Vitals metric, “Largest Contentful Paint”, which measures how long it takes for the largest visible element on the page to render. An oversized image therefore penalises both user experience and organic search ranking. On mobile, where most web traffic is concentrated today, this effect is even more pronounced: downloading an image designed for a computer screen only adds unnecessary weight, with no added value for the visitor.
Compression built into every step, not just at upload time
The solution isn’t to compress an image once in a while, but to establish a systematic discipline: resize images to their actual display size before compressing them, favour modern formats, and automate this step as much as possible rather than relying on everyone’s vigilance. Deferred loading of off-screen images (lazy loading) effectively complements this approach, by preventing every image on a page from loading simultaneously the moment a visitor arrives.
📋 Summary
| Identified problem | Concrete impact | Best practice |
|---|---|---|
| Uncompressed images | Slower loading, poor LCP | Systematic compression before upload |
| Outdated formats (JPEG/PNG everywhere) | Files heavier than they need to be | Migration to WebP or AVIF |
| Oversized dimensions | Unnecessary data downloads, especially on mobile | Resizing to actual display size |
