The author covers it: > JPEG recompression is the ability to losslessly re-encode JPEGs as JXL images while saving bits; the oft-cited number is 20% savings. H

The author covers it: > JPEG recompression is the ability to losslessly re-encode JPEGs as JXL images while saving bits; the oft-cited number is 20% savings. However, the user pays for this in decode time, as recompressed JPEGs take ~33% longer to decode