You cannot downgrade, but for what it's worth, I have a 5C on iOS 9.3.2 and its performance is sub-par. It isn't good. Is it acceptable? Yes. But not good. At least not compared to a myriad of both older and newer devices on initial versions of iOS. Battery life? Awful, and it has 85% of health with almost 700 cycles. So, battery health is incredible, but that has no reflection in battery life.
I have no idea what the performance is on iOS 10.