I believe Kirby creates responsive images (thumbnails) on the ‘fly’ as they are needed – if they have not already been created.
I’ve just implemented responsive images on my website. As non of the thumbs have been created the site is noticeably slow to load. Rather me manually visiting every page, at every different responsive image ‘breakpoint’, to create the thumbs – so that it is faster for future visitors – is it possible to create all of the thumbs in one go, perhaps before launching the site?