My daughter was getting ready to make bread pudding and she realized that the milk had gone bad. She asked me: “Dad, how quickly can you get me a gallon of milk?” The closest grocery store is about 10 minutes from my house, so I calculated the time to go back-and-forth plus the time needed at the store and told her 25 to 30 minutes.
On the way, I realized it was game day at the high school and unfortunately that meant a lot of traffic. What I thought would take ten minutes took 20 minutes one way. As I reached the store, I had more challenges such as finding a place to park and the long line at the counter. All in all, it took an extra 22 minutes.
By the time I reached home with the milk, 62 minutes had passed since my daughter made the request. Obviously, her experience was not great with that level of latency.
Latency is defined as the round trip time between making a request and receiving the response. In this case, the expected latency was 30 mins, but the actual latency was 62 mins.
Application performance is not any different. Application Developers build applications with amazing functionality with an expected latency in mind, however many factors influence the actual latency and in turn impact end user experience.