It turns out Apple’s mobile operating system includes a throttling mechanism for devices with weary batteries, designed to limit CPU utilization in order to prevent peak power demands that the battery is no longer capable of providing. In other words, the OS secretly stalls the CPU on older iPhones to stop them rapidly draining their aging batteries to zero.When the CPU runs slower, though, so do the apps on the device, and some of the iPhone users who experienced this first hand have been wondering why.

Source: Yes, your old iPhone is slowing down: iOS hits brakes on CPUs as batteries wear out • The Register