An original blockchain is necessarily the oldest. As long as a majority of work is dedicated to adding time to an original blockchain, it will always appear older than any competing counterfeit blockchain.

Work is energy over time, therefore proof-of-work is proof of time. Blocks are produced with work to represent time, and are added to a blockchain to increase its apparent age. The work required to produce a block must scale with the work capacity to maintain the block interval, so it's not about how fast the work is done, but about the time it takes to do it.

Energy efficiency in proof-of-work is measured, not by how many hashes can be done per second per watt, but by how few. The slower the hash function is, i.e. the more interrupts and memory accesses it involves that keeps the processor suspended and maximizes the delays between hashes, the more time and less energy it will consume. The idea is not to make the processor work harder between hashes, but to work less, if at all.