Hash rate : Algebraic Equation
The hash rate is literally how many distinct block header hashes you can try per second. Nothing more, nothing less. Note that I say "try" and not "compute". Determining whether a particular block header is interesting is somewhat faster than fully computing the hash:


You can drop the last few rounds of the hash implementation, and only compute the first 32 bits of the output (and if that those 32 bits are 0, recompute the hash in less optimized software to determine if it's a fully valid block).
You can exploit the fact that successive attempted block headers only differ in a few bytes, reusing part of the computation.
You can precompute things to find combinations of multiple block header attempts that share more parts of the evaluation than random ones (this is what AsicBoost is based on).


So determining the exact hashrate of certain hardware is a nontrivial engineering effort because of the above, and is complicated by things like architecture specific optimization, and things like exploiting special hardware instructions.

Such engineering has obviously not been done for iPhone, as it's a nonsensical target for computation. My back of the envelope computation suggests it could probably do a few MH/s, but as that still means ~50 million years to find a block, not much effort will be spent on this.