A hash function that runs a trillion times slower on the same hardware, would provide the same level of protection at a fraction of the cost.
It would not. Hash rate would be a lot less, but the cost in terms of energy would be the same.
The computational cost of a hash function is completely irrelevant. If you invent a hash function that is two times more computationally expensive, then you will simply drop the hash rate by half. Margin cost and profit remain the same, competition remains the same.