The weight of JPEG thumbnails, while varying less than PNG thumbnails', is often a relevant factor in the loading times observed on random Wikipedia pages, even with a fast connection. It may be even more important for MediaWikis using larger thumbnails.
https://github.com/mozilla/mozjpeg 2 was just released, see https://blog.mozilla.org/research/2014/03/05/introducing-the-mozjpeg-project/ and http://thenextweb.com/insider/2014/07/15/mozilla-releases-mozjpeg-2-0-facebook-tests-backs-jpeg-encoder-60000-donation/ for introduction, and should be relatively trivial to implement as "second pass" over JPEG thumbnails produced, right?
I know that VipsScaler is currently only used by Wikimedia wikis for PNGs (and that it's named after Vips), but would such an optimisation be in scope for this extension? If not, should we instead upstream a request to include a jpgcrush-like functionality in Vips; or use some other way?