Consider Burger King and McDonalds differentiated themselves a few decades ago: McDonalds maximized throughput with batch cooking, while Burger King relied on a sort of mechanized burger assembly line. That meant that Burger King's product was more consistent, and could be more easily customized, since they arrived in a continuous stream rather than all at once; that's why they emphasized that you could "have it your way". Among other things, this made labor quality less important, because the machine, not the worker, cooked the burgers.
But while the "burger stream" arguably delivered a better hamburger, it also meant throughput was bottlenecked; you got one burger every few seconds, no matter what. So Burger King tended to deliver a better burger during slow times, but McDonalds could handle the highest volume periods. In burgers, it turned out that volume mattered more than the ability to have exactly the burger you wanted.
In this case, Starbucks seems to be choosing quality--or at least, standardization of quality--over speed. As with Burger King, this is going to have some unwanted side effects.