[lrm242]

They are unable to do so. It is a queued system with a fixed transit latency through the queue of 350us. Any remaining un-executed of A is left on the IEX book.

IEX has this transit latency because they have order types that offer pegged execution, i.e. midpoints. A midpoint order is executed at the midpoint of the prevailing NBBO. The data required to build a view of the prevailing NBBO comes in from all other lit venues.

IEX has a 350us delay because they want to slow down the order entry side to give themselves enough time to make sure they are not pricing their pegged orders using stale market data. Simple as that.