The thing is, the logic to allow multiple instructions to execute in parallel in one thread safely is possibly costly. I don't know how much power that is. That is going to spoil the low power characteristics of these machines, and they would be wasting energy in order to get superb performance. On Intel Haswell and later new processors, if logic allows, you can execute four instructions in parallel, more where instructions get deleted altogether, register-to-register moves are an example. This crucifies ARM for straightforward computing tasks. Sometimes programs have sequential dependancies and no room for instruction parallelism and in that case your luck is out. But if your luck is in, you can easily get a three time or four times or more speed up, so your 2GHz processor turns into an 8GHz one effectively if you have the right kind of sequence of ordinary common instructions in an inner loop.
If ARM designers wish to get the very best battery performance then they simply will not deliver this and so they will always be three or four times slower in some fairly common situations. Or not. Mains powered machines, who cares, so there would be an argument for a massive gap between Mac CPUs and iPhone CPUs and who knows where you would put the iPad, probably in the iPhone category. However perhaps it would be an idea to give users a choice of iPad models depending on whether or not you, like me, never run your iPad on battery.