Also, I presume that "move" means "update the index, held in another object or objects" (no slight! If I understand correctly, that's what most on-disk filesyst

Also, I presume that "move" means "update the index, held in another object or objects" (no slight! If I understand correctly, that's what most on-disk filesystems do as well). That's the only way I can imagine getting performance that much better.