Question: Where specifically on the arm/hand should rapid picking speed come from?

Answer: 
i would say it's all in the flick of the wrist

the goal is that you don't really want to be straining your muscles much if at all. it's really about twitching the wrist quickly without flexing or constricting muscle much, to not tire your arm. you don't need strength for this. just speed.
A fast picking speed comes from mainly the hand both working to move the pick quickly. However the bicep does a little work, keeping the hand and arm steady to provide accurate picking.