This command will run jpegoptim in lossless mode for each .jpg file in a folder recursively. This is how I reduce the size of my photo & image collection.
find . \( -iname \*.jpg -or -iname \*.jpeg \) -print0 | xargs -P 4 -n 1 -0 jpegoptim
See How to use xargs in parallel for details on the -P 4 -n 1 syntax which runs 4 jpegoptim processes in parallel.
Note that the output may be scrambled due to 4 processes running in parallel, but it’s typically 4 times faster using this approach. You can also use
find . \( -iname \*.jpg -or -iname \*.jpeg \) -print0 | xargs -0 jpegoptim
to run just one jpegoptim process in parallel.