Don't fork a new process for each image, write something that uses the imagemagick library directly (or another library if you prefer). Don't do them serially,

Don't fork a new process for each image, write something that uses the imagemagick library directly (or another library if you prefer). Don't do them serially, use threads so you can make use of all your cores. That said, even serially 5 seconds per image seems very slow. Are you sure you're not hitting network latency from a remote filesystem or something? If so do some bulk copies to get the data locally then work on the local copy.