What's the issue?

A couple times now I've received reports that the Lighthouse performance audit is identifying “Defer offscreen images” as an opportunity for improvement… on AMP pages. The heuristic being used for lazy-loading images in Lighthouse is not matching up with AMP's lazy-loading implementation. These are AMP pages that are using amp-img.

Examples:

• https://www.ourtasteforlife.com/pink-beach-in-labuan-bajo/amp/
• https://elsoberano.org/ (an AMP-first site)

In the first case:

And in the second case:

In both cases, the images being reported as needing to be deferred are within 3 viewport heights from the top of the page. I recall that amp-img aggressively fetches images when they get within 3 viewports but heavier components like amp-youtube fetch within 1 viewport.

In any case, should the Lighthouse audit should be relaxed? Or should amp-img be more strict?

How do we reproduce the issue?

Run a Lighthouse performance audit on an AMP page that contains an amp-img in the second or third viewport from the top of the page.

What browsers are affected?

Lighthouse