[posnet]

HFT firms use a tiny fraction of the energy of proof of work crypto schemes. Even if you take into account energy requirements they used to build their high speed networks.

The main reason being is that they are all co-located with the exchange, and the byzantine fault tolerance is completely side stepped by the exchange having a single point of serialization between the HFT participants and the matching engine. And then just ensuring everyone has the same length cables to that single point of serialization.

So the policy of everyone connects through this single point, solves the fairness problem.

And then the fault tolerance is often just solved by having an active standby architecture, so if the primary matching engine goes down, you fail over to the backup.

You would be surprised at the low number of servers actually in the primary path for financial exchanges.