"Fully half"? The brain>mouth (or alternately the mouth>ears) thing includes the phrasing of the order, which takes aeons, and the brain>hands (or alternately the ears>brain) part includes figuring out what the order actually meant - plus, usually, wasting time with something like "Sure deal, boss!" that detracts from the handiwork. I'd say the nanoseconds part is more like one-third of the needlessly long chain at most.
Then again, it doesn't appear to matter. Whenever the skipper says "On my mark", things that realistically ought to call for split-second accuracy are nevertheless achieved. Perhaps the helmsman anticipates and ignores the "mark" altogether, relying instead on a computerized timer he sets according to the skipper's actual intentions?