Except the a neron doesn’t in of itself do anything with an isolated purpose as opposed to a machine code instruction in a CPU.
I mean a really really rough analogy (but, at least to me, easier to understand) is cars vs. people:
Cars do one thing, pretty much: Move forward by converting chemical energy in gasoline. People do all sorts of things, but most of them have at least some mechanical component.
My car gets 25 miles to the gallon. How many miles to the gallon does a human body get?
Really, the question is meaningless. You could try to extrapolate the caloric value of gasoline and apply that to walking, but that’s stretching.
Similarly, the human brain does all sorts of non-linear non-discrete analog processing of things outside and (more importantly) loopbacked inside the brain. It’s meaningless to even talk about absolute processing power, since the ‘programming’ is intrinsic to the ‘hardware’ or the neural structure. If you were to attempt to use the neural structure for maximum processing power, you’d have to reconfigure it destroying things like personality, vision, etc.
Also a particular neural network (what our brains seem to be, really) doesn’t really
have a reasonable linear equivalent. You can get functional equivalence through approximating it with a linear algorithm (Rice theorem, or something like that, it’s been a while), but the very structure of neural networks goes agains the whole idea of sequential steps of execution (and hence operating frequency, or even processing speed overall).