Is that something you're aware of? I suppose I now need to check if webp are actually consistently more often than not smaller than the lossy jpeg images, because if not then no point in using webp at all...
I know that Cloudflare also offers serving webp on their paid plans and does so selectively, i.e. only serves webp files if they are actually smaller than the source files. That would be a good feature for Robin.
In this case, it is clear why this happened. The Robin plugin tries to access the htaccess file to check if it exists, but iThemes has blocked this file. You can disable blocking in the iThemes settings.
The difference in the size of such images is small and does not affect the page loading speed, but site performance testing services ask that all images on the site be WebP.
@Temyk - the difference in size of the image I looked at was not that small.
JPG - 84.4 KB
Webp - 106 KB
Over 20KB difference for a single image is not insignificant in terms of performance optimization, especially if this happens across multiple images on a single page. That will affect page loading speed.
I know that performance testing services sometimes just expect all images to be WebP for their scoring, but then we shouldn't be doing performance optimization for these services but for the end user.