Re: I never understood why ...
No, it is to a design flaw. When batteries get old, the maximum rate at which you can draw power diminishes. It is a characteristic of the battery. Intrinsic in the technology. In order to stop the phone falling over Apple slows the clock speed while on maxim draw if and when the battery starts to fail. The phone is only 'slowed down' while it is drawing maximum from the battery.
It is an elegant and satisfactory solution that is only required by people who have abused their phones with excess overcharging. You can avoid even going there by removing your phone from the charger when it hits 100%.
Biting the hand that feeds IT
