Yes, but each image needs access to this 4GB (actually, I have no idea how much RAM it takes up), plus whatever the working set size is. It is a non-trivial ove

Yes, but each image needs access to this 4GB (actually, I have no idea how much RAM it takes up), plus whatever the working set size is. It is a non-trivial overhead that really limits throughput of your system, so you can process less images in parallel, so compressing billion of images in reasonable time suddenly may cost much more than the amount of storage it would save, compared to other methods.