Or the HFT shops use their own, creative secret formula? 1. Speed matters? If it is, the guy with highest speed wins and all others lose, from game theory point of view. So given the fact that multiple HFT shops exists, I think speed does not matter, or you should never put your strategy based on the assumption you are the FASTEST. 2. If speed does not matter, and I want to equity market making, with passive limit order at 3 times spread, will it be profitable? 3. Spread should be set according to order arrival rate and inventory. Assume order arrival rate is constant during the day, then the spread is only controlled by inventory, right? 4. Does paper "High-frequency trading in a limit order book" even work? I tried, but seems does not work. So that gives the question, if the classic theory does not work, how come HFT shops profit? Anyone like deep discussion with me are welcome. Thanks.