Donald R. J. Laming did a doctoral thesis in 1968 titled Information theory of choice-reaction times where part of the effort was to determine reaction times of human brain towards a stimulus. Reaction time has a been a favorite subject of experimental psychologists since the middle of the nineteenth century and there are many papers written of this topic.

R.J Laming determined that average reaction time for an observer to form a simple response to a stimulus is 220 ms. Part of this times is spent by the brain to detect the stimulus and the remaining time is spent to organize the response.

Recognition reaction time (time taken to recognize something) is longer than the simple reaction time towards a stimulus. The reason is of-course that for recognition to happen the brain has to run the process of understanding and comprehension. The average reaction time was measured to be 384ms

R.J Laming’s experiments clearly showed that as the context of recognition becomes more complicated (one symbol vs multiple symbol recognition) the recognition reaction time goes up.  It was established that with more complex context the recognition reaction time can go upto 550 – 750 ms. This interval of 550-750ms can be called the speed of thought 

Any interval longer than the speed-of-thought interval will be detected by the human brain as “waiting time” and the mind will start to wander to other topics which is a process that can’t really be controlled.

Of-course the longer the wait interval the further the mind is taken off task, eventually the user can become completely distracted. Once the response does come the user’s brain has to do a context switch , to remember where it was, what it wanted to do and so on. This context switching is an extremely tiring process.

By making sure your application response time is as close to being sub-second (thereby being close to speed-of-thought) as possible, you can keep the user’s engaged fully , making sure they are not distracted due to the wait and will not have to do mental context switches to come back into the task there were doing.

This is guaranteed to produce happy users, who get things done faster using the tools you provide them without feeling all mentally exhausted.

That is why i really think Speed of your application is, and will always be the killer feature. We are simply biologically programmed that way.