Serve Images in Next-Gen Formats with WebP

"Serve images in next-gen formats" is one of the most common Google PageSpeed and Lighthouse warnings. It means your JPG and PNG images could be smaller as WebP or AVIF, slowing the page. Converting to WebP is the direct fix, and it usually clears the audit while improving load time. This guide explains the warning and how to resolve it.

For the metrics behind it, see Core Web Vitals & Images. For the format, see What is WebP?.

What does "serve images in next-gen formats" mean?

The warning means PageSpeed found images that would be meaningfully smaller in a modern format like WebP or AVIF. Lighthouse compares each image's current size against an estimated WebP size and reports the potential saving in kilobytes. Large savings count against your performance score.

The audit is informational, not a penalty, but the underlying weight does slow the page and hurt Core Web Vitals. WebP is the fix it is pointing you toward.

Which formats count as "next-gen"?

PageSpeed treats WebP and AVIF as the next-generation formats to adopt. Both compress better than JPG and PNG. WebP is the practical default because of its near-universal browser support; AVIF compresses smaller where its slightly narrower support is acceptable.

The two sit in a preference chain — see WebP vs AVIF and Is AVIF Replacing WebP?.

How does WebP fix the PageSpeed warning?

Converting flagged images to WebP removes the wasted bytes the audit measured, usually clearing it. WebP at quality 80 is 25–34% smaller than the equivalent JPG, which is the saving Lighthouse estimated. Serve the WebP with a fallback so every browser still receives an image.

Convert the flagged images with JPG to WebP and PNG to WebP, then deliver them via the <picture> element or a CDN.

How do you verify the savings?

Re-run Lighthouse or PageSpeed Insights after converting and serving WebP. The "serve images in next-gen formats" audit should pass or show a far smaller potential saving, and the total page weight should drop. Check that the WebP is actually served by inspecting the image's response Content-Type.

If the audit still flags images, the originals are probably still being served — confirm the WebP variants are reaching the browser. Negotiation is covered in WebP and CDNs.

Does passing this audit improve SEO?

Passing the audit improves SEO indirectly by reducing page weight, which improves Core Web Vitals — a Google ranking signal. The audit score itself is not a ranking factor, but the faster load it represents is. Lighter images mean a faster Largest Contentful Paint, the metric WebP most affects.

The ranking link runs through performance, not format. See Image SEO Best Practices.

Where to go from here

The "next-gen formats" warning is PageSpeed telling you exactly where bytes are being wasted. Convert those images to WebP, serve them with a fallback, and the audit clears while the page gets faster.