One kind of efficiency which hasn't been talked about is the energy loss of things like state switching and keeping the current state enabled. I think that brains build on much more efficient primitives than the silicon transistors computer chips use and thus can perform far more computations for far less energy than a desktop CPU.
Another difference between CPUs and brains is that brains are much less general purpose. CPUs do run-time interpreting of instructions while brains process data in a more straightforward way like GPUs do. Many problems can be implemented into GPUs and they will run much faster. I'd argue that brains excel at such tasks while being harder at tasks that require lots of state to be kept around as well as conditional jumps like computing a hash function or compiling a program. CPUs excel at those tasks.
Another difference between CPUs and brains is that brains are much less general purpose. CPUs do run-time interpreting of instructions while brains process data in a more straightforward way like GPUs do. Many problems can be implemented into GPUs and they will run much faster. I'd argue that brains excel at such tasks while being harder at tasks that require lots of state to be kept around as well as conditional jumps like computing a hash function or compiling a program. CPUs excel at those tasks.