This particular client has a few hundred thousand images and tens of thousands of animated gifs.

It's the animated gifs that take forever because they have to be processed with gifsicle and then converted to mpeg-4.

Imgix does all of that. I don't know what kind of voodoo they are doing on the gif crop, but it's lightning fast.

This might be more common than you think. I worked at a company a few years ago with ~50 sizes of each image. When we wanted to launch a new page layout (or iOS app, or what have you), it literally required several weeks to render the millions and millions of images to that new size.

And of course things didn't work 100%, since some of the originals had mysteriously disappeared or rotted away. Not to mention the on-going storage costs of these 51 copies of each image, the vast majority of which were never requested even a single time.

Another approach at a different company was to have thousands of EC2 instances running 24/7 to resize images as users uploaded them and another few thousand to handle the backlog that the first group couldn't get to within their time budget. It got the job done, but it was pretty expensive and time consuming to babysit.

2 ways to do thumbnails. 1 Precompute(what they did) and 2 Compute on demand(imgix)

Of course the best is to do something in the middle.