If a broker's feed lags the underlying market by even a few milliseconds, a fast system can buy at a price the market has already left behind. It is a genuine flaw in the broker's plumbing rather than an edge in the market.
Brokers respond with last-look, minimum hold times, execution delays and clauses in their terms allowing them to void trades they deem abusive. Many retail account agreements explicitly prohibit exploiting feed latency, and profits from it are routinely reversed.
Example: a real EUR/USD print moves to 1.0846 while a broker still shows 1.0840 for 80 milliseconds. An automated order buys at 1.0840 and closes at 1.0846 for $60 per lot, repeated hundreds of times a day until the account is restricted.
Related: last-look, expert-advisor, triangular-arbitrage, best-execution