Go read the website I linked to. NASDAQ publishes their methodology. I'll quote it for you here: OUCH Measurements of Order Latency * The time it takes

Go read the website I linked to. NASDAQ publishes their methodology. I'll quote it for you here: OUCH Measurements of Order Latency * The time it takes to accept, process, and acknowledge or fill an order. * Measured as the complete round trip for an order or cancel transaction leaving a customer network port, through the co-lo network and application firewalls to the matching engine and back. * Measured in the production environment during a full trading day including opening and close. As you can see, they are including the time it takes to acknowledge and fill the order. It is the complete round trip time, and does include transit latency through the co-located network (which I thought was excluded).