The lack of efficiency in the brain is a trade off for adaptability. There's no doubt that we can transmit signals faster than neurons can fire but the cost is drastically reduced adaptation. There are micro, meso, and macroscopic networks in the brain that have different degrees of "adaptability". This isn't even considering the variety in neurons or the additional signalling cascades by non neuronal tissue. How does all of this contribute to intelligence? We don't know exactly but much of this has survived millions of years of evolution in many animals so it probably has some role.

That's not to say we can't do this with computers and less computational power. However, it's really improbable that a couple layers of adaptability on an artificial neuron network will be anywhere near sufficient to simulate intelligence in even a rodent.