It's theoretically possible and has always been for somebody to see order execution in progress and trade ahead of it. In practice, that's frequently just a sid

It's theoretically possible and has always been for somebody to see order execution in progress and trade ahead of it. In practice, that's frequently just a side effect of somebody being so slow that their actions trigger quant algorithms, it's not a super profitable game trying to latency arb proper market sweeps anymore (not to say latency is not important in but, but it's usually for other reasons)