JPEG minification with mozjpeg
tl;dr: Use mozjpeg
to minify JPEGs.
brew install mozjpeg
wget https://upload.wikimedia.org/wikipedia/commons/a/a9/Example.jpg
cjpeg -outfile Example.min.jpg -quality 90 Example.jpg
8.8K Example.jpg
6.7K Example.min.jpg
The mozjpeg tool was able to optimize the file by (8.8 - 6.7) / 8.8 * 100 = 24%
; this is a material improvement.