The variances observed between otherwise identical hardware and allegedly identical OS and application software are due to the variances in installed and running app. Different installed apps will cause differences in competition for scarce RAM and CPU resources.
iOS is a derivative of BSD Unix. It looks like that much of the problem is due to how it handles (or mishandles) demands for RAM that can't be satisfied. Traditionally, when a Unix process (app) needed more memory and there wasn't enough available, the memory image of one of the idle processes would get temporarily copied to the hard disk (called the swap area or backing store.) But because Apple tries to limit the number of writes it makes to flash memory, iOS doesn't support backing store. If one app needs memory, iOS will request other running apps to free some of their memory up. If not enough memory is available that way, it starts to kill processes, go through their pockets for loose change and memory, and handing it over to the process that asked for memory.
More details here:
https://developer.apple.com/library...tual/ManagingMemory/Articles/AboutMemory.html
Edit: Bottom line is when comparing mobile devices that use flash memory for persistent storage is to favor those with the largest amount of RAM.