If you’re optimizing for the Web Vitals, you might be interested to hear that images account for ~42% of the Largest Contentful Paint element for websites. Key user-centric metrics often depend on the size, number, layout, and loading priority of images on the page. This is why a lot of our guidance on performance talks about image optimization.

In this article, we’ll highlight how modern image formats (AVIF or WebP) can improve compression by up to 50% and deliver better quality per-byte while still looking visually appealing. We’ll compare what’s possible at high-quality, low-quality and file-size targets.

A tl;dr of recommendations can be found below:

• AVIF is a solid first choice if lossy, low-fidelity compression is acceptable and saving bandwidth is the number one priority. Assuming encode/decode speeds meet your needs.
• WebP is more widely supported and may be used for rendering regular images where advanced features like wide color gamut or text overlays are not required.
• AVIF may not be able to compress non-photographic images as well as PNG or lossless WebP. Compression savings from WebP may be lower than JPEG for high-fidelity lossy compression.
• If both AVIF and WebP are not viable options, consider evaluating MozJPEG (optimize JPEG images), OxiPNG (non-photographic images), or JPEG 2000 (lossy or lossless photographic images).
• Progressive enhancement via <picture> lets the browser choose the first supported format in the order of preference. This implementation is considerably simplified when using image CDN’s where the Accept Header and content negotiation (e.g. auto-format and quality) can serve the best image.