Network Latency
Network latency, often referred to as network delay, is the time required for data to travel from the sender to the receiver. Network latency is different from network speed or bandwidth or throughput, which is the amount of data that can be transferred per unit time. A network can be both high speed and high latency. For example, a 100 Mbps network speed means the network can transfer 100M bits of data per second, while a 500ms network latency means that the data will need 500 ms to reach the receiver.

One way network latency is difficult to measure. Usually, Ping is used to measure the round-trip time instead. The round-trip time is the sum of the network latencies for the transmit path, the receive path, and the echo request processing time of the device being pinged. Note that the transmit and receive paths are not guaranteed to be the same path, although in practice they are usually the same. Even if they are the same path, the transmit and receive latencies can be different. It is like traffic on a road - it can be congested in one direction and running smoothly in the opposite direction. Some network connections, such as ADSL, can have different bandwidth for different directions.
What Causes Network Latency?
Effects of Network Latency
Using Network Latency Measurement for Network Tuning
Many network issues, such as network congestion, affects both network performance and network latency. Network performance is difficult to measure. You may need to transmit large amount of data as fast as possible to saturate the network in order to determine its performance. The test is disruptive - it affects other users of the network. It is also difficult to obtain real-time readings, that is, obtain second by second variation in network performance. On the other hand, measuring real-time network latency is easy with Fast Ping. So network latency is often used as a relative indicator for network trouble-shooting and performance tuning.

As a relative indicator, the network latency of a network path can be compared against that of the same path at a different time. For example, if changing the position of a Wi-Fi antenna reduces network latency of a network path, we can conclude that it improves network performance.