This is intended to correspond to Audionautics' answer, insofar as it uses Python's multiprocessing.

This is intended as an improvement to Jakob's answer, wherein the image is loaded just once and then resized down three times instead of re-loading each time to produce each new resolution. Method 6 - Sequential PIL with single load & successive resize #!/usr/local/bin/python3 import glob from PIL import Image sizes = files = glob.glob('image*.jpg') N=0 for image in files: for size in sizes: im=Image.open(image) im.thumbnail(size) im.save('t-%d-%s.jpg' % (N,size)) N=N+1 This is intended to correspond to Jakob's answer. #!/bin/bash start=$SECONDS # Loop over all files for f in image*.jpg do # Loop over all sizes for s in 1600 720 120 do echo Reducing $f to $ ::: *.jpg echo Time: $((SECONDS-start)) Then it is read again and a different sized thumbnail is produced. Each image is read and a thumbnail is produced. Here are the techniques and results - each in its own section. I collected together 1000 high resolution 12MP iPhone 6S images, each 4032x3024 pixels and use an 8-core iMac. I fancied some fun so I did some benchmarking on the various methods suggested above and a few ideas of my own.

Python imagemagick