No need to slow the order flow down. Just make the system transactional (in a computer science sense, see
https://en.wikipedia.org/wiki/Transaction_processing_system).
Such a system would not allow privileged agents to view incoming orders before they have been either transacted (processed) or put on an order queue.
In such a system speed is irrelevant. Once an order is received, and there is existing opposite order(s) then the transaction will occur before anything else happens. No opportunity for HFT to sneakily remove or modify orders.
I find it very difficult to understand why **promotion blocked** would be the ONLY financial transfer system in Australia that is not transactionally safe, unless there may be some financial benefit to the ASX. A rather less cynical view would be that the ASX, a very long time ago, bought some rather shoddy and third rate software from USA. But that is no reason why the aberrant behaviour should continue until today.
