should I let my sourdough loaf proof in the fridge or on the counter?
Speed is not the point. Speed is the opposite of the point. Refrigerator rising makes for a long, slow rise and flavor development - it's not unique to sourdough, it's very common in pizza dough (though not usually the "final rise" in pizza, as that would be very difficult to arrange enough fridge space for.)

You can do it, or not, or try both and compare.