Here's your nitty-gritty: If a participant places a large buy order into the IEX, this is going to affect the price of that asset, both at the IEX (obviously)

Here's your nitty-gritty: If a participant places a large buy order into the IEX, this is going to affect the price of that asset, both at the IEX (obviously) and anywhere else it is listed, along with any correlated assets. Now, the original participant will have bought its shares at the price it wanted at the IEX and be happy about it. However, market makers at other exchanges will not be aware of this upward price shock just yet. They will be blindly quoting sell offers at a price that had just been bought in bulk at the IEX. Whoever can receive the information out of the IEX 'magic shoebox' first and shoot that information over to the other exchanges will be able to make a profit. You can see that it doesn't matter how long the IEX delay is. The original buyer will be the first to know of course, but they are unlikely to want to take on the risk of all available shares at that price at all exchanges. Thus, there will likely be some left for HFT to take.