Latency is the transmission delay added by the processing of the device in question. For example, a firewall examines incoming data and applies some set of rule to determine if the data should pass through or be blocked. The amount of time it takes the firewall to decide this and then send it on its way is called the latency time. Unfortunately, there is no easy way to determine the latency of a particular security product or for any product. Latency is a function of the processing power of the CPU, the Bus speed, the memory chip speed, the overall architecture of the product and the amount of processing that needs to be done. Some manufacturers make claims about the latency based on particular configurations. One of the fastest I've heard of is the CloudShield device that can act as a firewall on circuits up to OC-48. The claimed latency for that device is 6 milliseconds. However, until tested in a controlled laboratory environment, it would be very difficult to determine what latency would be added by inserting a particular product into a network configuration.