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.