Elon Musk on How to Build the Future

Why do they discuss the speed of the line? Isn't the finalized output a much more useful metric? (e.g. one car per hour)?

If there's a ton of work done at the various checkpoints, the pace is slow but maybe the length of the line isn't very far?

The speed of the line is important because it is directly tied to the output in the form of the "pipeline". The question then become how long is the pipeline? 1day, 2days, 20 weeks? In which case you wait x-amount of time for the first car and then the rest is gravy.