You decide you are better off without the application service than waiting the amount of time it requires
You feel your machine is lingering to keep the application running
You start receiving inaccurate responses after having used the application for so long
Out-of-memory exceptions start popping up on the screen
Efficiency is a nonfunctional attribute that characterizes the system. It is demonstrated by the latter's consumption for time and resources when subjected to an increasing load. Therefore, an efficient system is one that provides responses while abiding by the predefined limits for time and resource usage.
Testing efficiency is not achieved by applying an arbitrary simultaneous weight of users over the system; neither is it confined to varying their numbers. Actually, lots of other factors play a role in many variations of efficiency...