Order priority. Some exchanges pro-rata trade matching (i.e. you'll get a percentage of your order filled), others just use a FIFO queue. So if the price chan

Order priority. Some exchanges pro-rata trade matching (i.e. you'll get a percentage of your order filled), others just use a FIFO queue. So if the price changes to something that want to buy/sell at, and you get in first, you get matched, your order is filled, you win. Speed of light is 1ft per nanosecond. So 1000 ft of network cable between you and the exchange server, is 1 microsecond of latency, just on the wire. Hence straight-line microwave towers, or co-located servers "in the same cabinet". Then you have the latency from the wire to machine memory, so there's a whole industry of Solarflare-esque network cards that work entirely in userland to further reduce latency.