You can still brute force it, which is more or less how back propagation works.
Intractable problems of that scale can’t be brute forced because the brute force solution can’t be run within the time scale of the universe, using the resources of the universe. If we’re talking about maintaining all the computing power of humanity towards a solution and hoping to solve it before the sun expands to cover the earth in about 7.5 billion years, then it’s not a real solution.
Not exactly. More the other way around: that human behaviors in response to inputs are only observed to process about 10 bits per second, so it is fair to conclude that brains are highly restricting the parameters of the information that actually gets used and processed.
When you require the brain to process more information and discard less, it forces the brain to slow down, and the observed rate of speed is on the scale of 5-40 bits per second, depending on the task.