You're correct, blockchain-based solutions are unlikely to be able to support HFT, even in the medium term. However, OT is decentralized and can handle HFT just fine (amongst other things). The ideal setup would be decentralized exchanges operating their 'backbones' on a blockchain and frontends (including HFT) over OT.
after this comment I would have expected a link to an elaborate report, of how exactly OT handles anything at all, let alone sub microsecond precision.