OK - The only 'simple' plugin I found that will do this, and also works with Windows and IIS, as it uses the PHP plugin libraries. Great work! It only failed to convert 2 of several hundred images. What the author does not make clear (to me), this plugin DOES alter your site to serve the .webp images, too. Perfect! My only issue was that the quality slider seem to have no effect at all, and the quality of the generated .webp images is a little less than I would have liked (less than the .jpegs), but, yes, everything is smaller! Note - On IIS, you will need to add the MIME type to support serving .webp. Good job.
Update: One issue I found. Wordpress does not like the sessions created by this plugin. You will get the error "A PHP session was created by a session_start() function call. This interferes with REST API and loopback requests. The session should be closed by session_write_close() before making any HTTP requests."I think the plugin probably needs to be updated to comply.