Some statistics. TL;DR Lepton indeed provides about 22% of size savings. When running Lepton through JPEG photos downloaded from Flickr, I've got about 23.37%

Some statistics. TL;DR Lepton indeed provides about 22% of size savings. When running Lepton through JPEG photos downloaded from Flickr, I've got about 23.37% of size savings. When running Lepton through JPEGs generated by mozjpeg (default settings: -quality 75, progressive) from JPEG photos download from Flickr, I've got 22.63% of size savings. The mozjpeg output is about 3.83 times smaller than the original JPEG photo, on average.