These mobile processors are very aggressive in lowering power demand when there is little computational load. Then, when you open an app or give it anything to

These mobile processors are very aggressive in lowering power demand when there is little computational load. Then, when you open an app or give it anything to do at all, power consumption jumps by orders of magnitude. An aged battery will show big voltage drops under load. What was looking like a fine and dandy voltage (voltage is used to infer state of charge) can suddenly plummet. It can get so bad that your phone might even shut down because it starts to undervolt. So, choosing between the lesser of two evils, iOS throttles down max. power draw, and thus max. processing speed, when the battery ages. The alternative would be random shutdowns, or your battery jumping from 80 to 8% suddenly. In settings -> battery -> health & charging you can check in which regime you are.