Response time is generally defined as the duration of time needed by the system to express state changes to the user.
In general, short durations and instantaneous response times are desirable.
Instantaneous means that the user perceives system reactions as immediate.
But even in situations in which an instantaneous response cannot be obtained, there must be some indication to the user that the system has received the request for action and is working on a response.
As significant as absolute response time is response time stability. Response time stability covers the invariance of the duration for identical or similar computational resources.
For example, pull-down menus are expected to pop up instantaneously as soon as a mouse button is pressed. Variations in response time will impede anticipation exploited by motor skill.