The reason is that "the market" is made up multiple exchanges and to make a trade you need to do it on a specific exchange, but stocks are spread across all of the exchanges so if you need to make a big trade you have to send the trade order to several exchanges at once.
The problem is that HFT firms will see the trade hit the closest exchange (in terms of network latency), and then rush out to buy out all of the remaining shares of that stock at that price on the other exchanges (beating you because they HFT better) and relisting them at a higher price.
HFT firms will tell you that this is perfectly acceptable behavior because if you then buy the stock from them that the original person simply wasn't charging enough for it. They will also tell you that it improves liquidity, but that doesn't make sense either. It's not like they're sitting on the shares keeping the markets moving. All it does is add a tax to trades.
IEX delays the transactions just enough that the nanosecond advantages that HFT enjoys can't be used to front run trades.
The problem is that HFT firms will see the trade hit the closest exchange (in terms of network latency), and then rush out to buy out all of the remaining shares of that stock at that price on the other exchanges (beating you because they HFT better) and relisting them at a higher price.
HFT firms will tell you that this is perfectly acceptable behavior because if you then buy the stock from them that the original person simply wasn't charging enough for it. They will also tell you that it improves liquidity, but that doesn't make sense either. It's not like they're sitting on the shares keeping the markets moving. All it does is add a tax to trades.
IEX delays the transactions just enough that the nanosecond advantages that HFT enjoys can't be used to front run trades.