Its-Just-Nans

First idea was to use https://github.com/google/guetzli but it is known for slow performance

Then you found https://github.com/mozilla/mozjpeg and build+install it

cd /tmp
git clone https://github.com/mozilla/mozjpeg.git
cd mozjpeg
mkdir build && cd build
cmake -G"Unix Makefiles" ../

# you need sudo to install at /opt/mozjpeg/bin/cjpeg
sudo make install

# now make them available in your PATH
cd /usr/local/bin
sudo ln -s /opt/mozjpeg/bin/cjpeg
sudo ln -s /opt/mozjpeg/bin/djpeg
sudo ln -s /opt/mozjpeg/bin/cjpeg mozjpeg

The usage command is simple

mozjpeg -quality 90 -outfile output.jpg input.jpg

And then, using our favorite tool fd

# for all jpg extension files, compress them
fd -e jpg -j 10 -x mozjpeg -quality 90 -outfile {.}_c.jpg {}
mkdir compressed/
mv *_c.jpg compressed/

# for other format (extension)
fd -e png -j 10 -x mozjpeg -quality 90 -outfile {.}.jpg {}
# then delete png files

Other useful commands

# remove all NOT compressed files
fd -e jpg -E "*_c.jpg" -j 10 -x rm {}

# rename all compressed files
fd --glob "*_c.jpg" -j 10 -x rename 's/_c\.jpg$/.jpg/' {}