Special bots, searching for arbitrage on decentralised exchanges get priority ordering and make money out of it.

Firms that deploy autonomous trading programs are able to get priority ordering by paying higher fees. Bots use trades where orders from other traders are visible and they manage to take advantage through practices such as front running. This unethical activity is well-known among experienced exchange traders.

Still just a small fraction of overall trading volume is traded on decentralized exchanges. We can expect decentralized exchanges to build out their own decentralized versions in sequence. In the same way, we expect the trend of rising trading volume on decentralized exchanges.

Moreover, similar aggressive activities most likely take place on centralized exchanges too. Experts suggest, the scope of such activities can be in billions of dollars. It's always a very challenging task to detect high-frequency trading, because it requires much time, data and consensus upon what is and what isn't illegal activity. Often development of a new autonomous software is needed.