> Since these URLs are private, Google does not want to store them anywhere permanently in the Google servers.

Yeah, I read that part, I just don't understand it. They are obviously storing the contents of the image somewhere (in memory?) for an hour, otherwise there's no point prefetching it in the first place. At the end of that hour, they download it again. If the image isn't different, they would be better off not throwing away the old copy.

They should've at least implemented lazy loading, i.e only fetch the thumbs the user is looking at.