latency
How long you wait between sending a request and getting the answer back.
Latency is the delay a user or system feels, usually measured in milliseconds. A thousand milliseconds make one second. Low latency matters when an answer is needed while someone waits, such as inside a live chat or at a checkout. High latency is often fine for work done overnight in the background.
Think of it as
Like the pause between asking a shop assistant a question and hearing the answer. Two seconds feels fine; twenty seconds and the customer walks away.
Example
A fraud check that takes 30 milliseconds can run while a card payment is processing. One that takes five seconds cannot, because the customer at the till will not wait that long.