Thanks, David ... it took maybe 30 or 45 seconds as compared with about 12-15 before, so I'll agree that even on my broadband it couldn't have been anywhere near the full file.
Hopefully this is just one of those glitches in the updating process (of which there still seem to be quite a few) which Alwil's still working on, and we'll see a fix soon. In the meantime, I'd probably be willing to gamble that it's OK to manually delete the older set, unless maybe it's being kept for insurance till they get the updater bugs worked out.
(Edit) Be interesting to see whether such files and folders continue to accumulate, or if what's kept is just the current one plus the previous one. We should know soon enough, if they maintain their twice a day routine.