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

Answer: 
The pick type makes a great difference to me - if I'm chugging I'll use a fairly thick pick, but for speed I'll go for a 0.4 or 0.45 mm pick, and as new as possible...if it is worn you have to move it further to get the point of the pick past the string to return back across it.
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.