Earlier quoted context omitted.
Fetch and save operations get pretty slow with even just 10,000 entities (if you touch all those entities). It gets a little better if you pre-fetch all the entities rather than one at a time, but the save is still very slow. Breaking that out into a backround thread is feasible, but as the previous commenter mentions, then you risk momentary divergence between the UI and the backend.
Index properly. Save in a queue.
This is fetching every entity by its indexed key. My understanding of saving in a background queue is that during that 1-2 seconds, the ui will still be using the pre-modification data set, so very briefly the ui may appear incorrect, no? Not a huge deal, but if the ui will be wrong, then for our app that would still force us to pause the ui until the save finishes.