How PageSpeed Insights Evaluates AVIF Images

Google PageSpeed Insights assesses web pages based on load efficiency, payload size, and user experience metrics, viewing modern image formats like AVIF as a primary optimization best practice. By transitioning from legacy formats such as JPEG and PNG to AVIF, websites directly satisfy audit requirements for modern image delivery, reduce overall transfer weights, and improve critical Core Web Vitals metrics like Largest Contentful Paint (LCP).

The "Serve Images in Next-Gen Formats" Audit

PageSpeed Insights runs on Google Lighthouse, which features a specific performance audit titled "Serve images in next-gen formats." When an audit is triggered, Lighthouse scans all image assets on a page and simulates converting them to AVIF and WebP. If the potential data savings exceed a threshold of 8 KiB, the audit flags the page with an optimization opportunity. Delivering images in AVIF satisfies this check, clearing the warning and contributing positively to the overall performance score.

Direct Impact on Largest Contentful Paint (LCP)

The most significant scoring advantage of using AVIF relates to Largest Contentful Paint (LCP), a Core Web Vitals metric accounting for 25% of the overall Lighthouse performance score. On many websites, the LCP element is a featured hero image or banner. Because AVIF provides compression that is typically 20% to 50% more efficient than JPEG or WebP without noticeable fidelity loss, the download duration of the LCP element drops substantially, directly reducing the page's LCP time.

Chromium-Based Evaluation Environment

PageSpeed Insights evaluates web pages using a headless Chromium browser. Because Chromium has native support for AVIF decoding, the testing tool directly requests and downloads the AVIF variant when it is offered via content negotiation (Accept headers) or through the HTML <picture> element. The tool evaluates the real byte transfer of the AVIF file rather than an estimation, rewarding the site for the actual reduction in network payload.

Handling Image Sizing Alongside Modern Formats

While AVIF optimizes compression efficiency, PageSpeed Insights still evaluates responsive display sizing. The "Properly size images" audit runs independently of format choice. If a page serves an AVIF image with desktop dimensions to a mobile viewport, PageSpeed Insights will still flag the resource as oversized. Delivering properly scaled responsive variants using the <picture> tag or srcset attribute ensures full audit compliance.

Best Practice: Implementation for Full Scoring Benefits

To maximize results in PageSpeed Insights while maintaining universal browser compatibility, deliver AVIF using the standard <picture> pattern:

<picture>
  <source srcset="image.avif" type="image/avif">
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" alt="Description" width="800" height="600" loading="lazy">
</picture>

PageSpeed Insights detects the AVIF source, downloads the reduced payload, eliminates next-gen image warnings, and reflects the resulting latency reductions across all network-dependent performance metrics.