The idea is to offer an option that will allow to generate thumbnails based on a chain. I.e. a given thumbnail would be generated based on a bigger thumbnail rather than on the original whenever possible. This should greatly increase performance for large files, and if good bucket values are picked, the visual impact should be unnoticeable.
I verified that the visual impact would be minimal with a power-of-2 progression with chaining up to 5 thumbnails by running an informal survey which I invited developers and commons users to participate in.