Question regarding alter HTML

First: Have you tried both methods of Alter HTML ? (I mean both “Replace tags with <picture> tags, adding the webp to srcset.” and “Replace image URLs”).

The Alter HTML feature was primarily created for those case where serving varied image responses was a problem (ie on Nginx without access to server config or when on CDN which cannot be set up to allow two different types on same URL).

An extra bonus of alter HTML is that when users right-click the image and save it to disk, it will have the correct extension. With varied image responses, on webp-enabled browsers, there is a discrepancy between file extension and image type.

Performance-wise, the gain of enabling Alter HTML in “Varied image responses” mode is insignificant and hardly measurable.

Actually, if you are not using page caching, it will be better performance-wise to disable Alter HTML as the Alter HTML feature increases the “time to first byte” (TTFB). Especially on pages with many images on it.

I shall then recommend that you disable “Alter HTML”. As I said, the option does not get you better performance, but might get you worse.

I shall also recommend that you convert all images with Bulk convert. After the bulk conversion, the webp’s will be delivered directly (make sure you enable “Enable direct redirection to existing converted images?”). In addition to this, I recommend you enable “Convert on upload”. Uploading will take a bit longer, but it is worth it, because it prevents the situation you describe where you have 14 unconverted images on a page.