That sprocket DOES NOT slow the cross auger down. It has NOTHING to do with the cross auger. That larger sprocket speeds up the feeder house chain. The power transfer on a JD feeder house is down the left side and across the bottom and back up the right side to run the feeder house chain.
Your JD 4420 would be exactly like that too. The small sprocket is for corn and the larger one is for small grains. I usually just leave mine on the larger one.