This is a good subject.
I gave this point some thought a few years back & decided I'd carry spare mags in rear/back hip(right side). I'm left handed & would want to maintain the same draw/reload method for duty-uniform details as a EP or concealed carry(off duty) draw.
The back-rear carry method would conceal the spare mags but still have a swift smooth motion.
Muscle memory & reflex are common too. You won't shift around looking for magazines in a high stress event. ;
Every 2nd matters.