Can you try an experiment for me (if you don't have anything useful on the
array yet)?

Stop the array.  Recreate it exactly the same way as before, and see if it
goes faster.  I think it might - at least until it got up to where it was up
to.

Um, unfortunately, I do (I didn't actually make the posting until after
the sync finished), but it should actually be non-destructive to do that
to an existing array, right?

It's just like deleting and re-creating a partition; as long as you're
*damn* sure you kept the parameters the same, it's a no-op.  Right?

I still have the data on the backup drives, so it's "only" a day's
lost work if I fry it (I did some reorganization as well as simple bulk
copying), so it's not *deadly* dangerous...

Thank you for the response!