The reasons why in case we decide to issue a lot of cranks
to be manufactured, it should take much less time than the prototype did,
are exactly the reverse of why it took so long
to make the prototype:
special tools are already made
special procedures are already determined
if we make a lot of parts all together (the more, the better) we do not need to be synchronized with Their own production anymore, the production of a batch would start "immediately" and advance independently from Theirs
no personal favours to ask: we pay, a batch of parts is made (well, sort of: it is still a personal favour, but easier)
results would be even better
as running a batch would make sense to spend more time in a even more accurate setup of the machines, and allow for more wastes in the process.
Similar considerations do apply to any other part we wish: special tools are not already made but in any case, if we ask to run a batch of... "anything" it should be done much faster, cheaper and better than if I ask to do a one-off piece as a favour.