A: Your premise is in error. Independent testing has been performed that determined there isn’t any code within newer iOS systems that makes older units slower.
However, as with almost every OS and app upgrade, there are significantly more features and functions (most under the hood), requiring more cpu cycles. This can have the effect of slowing down the perceived performance of the device. (Perceived, because the device is still cranking the same number of instructions per second, but there are now more instructions to process.)
This is why my old PC with a 60MHz Pentium, which was a screaming rocket ship when running DOS, would be impossibly slow running Windows 10 - and why it is subjectively just as fast (when running old DOS) as the new 4GHz multi-cores running Win 10.