There is barely any software in the hot path of competitive equity HFTs these days. Even the most finely tuned C++ is probably on the order of single digit micros tick to trade, but the goal is sub-micro.
There's a huge amount of software trading in the 1-10us range, probably more than pure hardware. Fpgas often play a role in these stacks as market data parsing/disseminating and order entry however.
The quickest in terms of tick-to-trade (MD received to trade) are certainly all hardware, but having better connectivity changes the game by a lot. Latency arb trades generally aren't that great, the most competitive in terms of PnL don't depend on (but usually utilize) tick-to-trade style speed.
