I guess in modern terms "front running" really just means getting arbed. If someone fast sees prices move in one market, they can buy at a relatively unfair pri

I guess in modern terms "front running" really just means getting arbed. If someone fast sees prices move in one market, they can buy at a relatively unfair price elsewhere, even sometimes outrunning the person who moved prices in the first place. That's not really a bad thing. It makes prices move together and spreads out liquidity across markets. It's also not risk-free money. Even a pure arb has legging risk. A lot of dark pools have orders "pegged" to the mid-market of the best bid and offer. For example if a stock is $100.01 bid offered at $100.02, the order would float at $100.015 and adjust as the price moves. If the market moves rapidly, and a fast trader sees it go $100.02 bid offered at $100.03, sometimes they can trade with the pegged order at $100.015 before the system knows the price even moved. Make a half-penny, rinse, repeat a million times. At least that's the story. IEX prevents some of this by computing their reference price in real-time that's used to determine where orders can execute and where pegged orders are priced, while delaying incoming orders by 350 microseconds so nobody can outrun their calculation of the best bid and offer to pick off stale orders. Mind you, this isn't even a real issue in markets today. There are a million ways to trade orders that are statistically mispriced before the market actually moves, so everyone will be taking them with these predictions with little left for the guys doing a pure arb. If you blindly peg your order to any benchmark, you are going to lose. Also, there was already a market solution for this. Off-exchange markets have discretion on who they allow in and who can interact with one another. If you do nothing but arb slow orders in their pool, they'll either kick you out or restrict who you can trade with.