This plugin converts images (or maybe more correctly converts and “serves”) to the WebP format.
I uploaded the above image in jpeg format and also used it as the featured image (so the image shows as the thumbnail on the home page). Right click on the image above and choose “Save image as…” and you’ll see that it saves on your hard drive with the .webp extension. The original image was 1.6MB and the webp version is 216KB – quite the savings and it will correspondingly speed up the display of the image on the website.
To do this conversion I used the plugin called WebP Converter for Media by Mateusz Gbiorczyk.