Because we can see that using normal computational methods to even approximate the behavior that seems to arise for consciousness, th epower consumption is orders of magnitude larger. Additionally, the apparent clock rate of the brain is insufficient in the same comparison.How are you determining sufficient power consumption? If we take the upper bound of computational power usage as the black hole threshold (Bekenstein bound), and the lower bound as the Landauer limit, then I think we can all agree that human brains are nowhere near either limit.
In terms of time, I've never heard of a lower bound or minimum clock speed condition. (I suppose the potential heat death of the universe is an effective lower bound on clock speed.) But given that human brains can indeed compute, it would seem that our clock speeds satisfy any possible minimum requirements.