It's not about the time it takes to resize a new image, it's about the time it takes to change _all_ of your images when design changes happen. a preprocess-on

It's not about the time it takes to resize a new image, it's about the time it takes to change _all_ of your images when design changes happen. a preprocess-only system means that you need to have a migration system in place, and it means that your designers have just a little more overhead to make tweaks. It's manageable, but producing the images on demand makes for a more easily changed system with fewer moving parts.