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What is the shortest perceivable application response delay?
I’ve been profiling some JavaScript UI code because it feels a little laggy. So far, I’ve found some bottlenecks and optimized them out, but I’d like to define a measurable requirement for this.
How quickly should a response occur in order for a human not to notice lag? For example, what’s the minimum detectable delay between when a keyboard key is pressed and when a letter appears on the screen? At what point is further optimization not going to make any difference to a human?
A lot of monitors have a refresh rate at about in the 60-120Hz range. Does that mean the magic number is around 8-16ms?
Considering the “press key” event and the letter appearing on the screen as two separate frames, means that, if the user presses a key while looking at the screen, he will want to see it exactly afterwards. This “exactly afterwards” means it should have a 60 Hz response time or higher.
For this reason, a 8-16 ms value should indeed be aimed for, since it will result in the same effect one sees in movies. In other words, the user will have no perception of delay for such values.
However, you must keep in mind that the keyboard has a polling time of its own, and that additional delays not necessarily connected with the script itself may interfere in its time. For those reasons aiming for values higher than 60 Hz will give you a bigger safety margin against those other possible influences that may add a minor delay.
Also of notice is the fact that in some applications, a delay of 100 ms might seem unnoticeable, but it is in fact noticeable since it corresponds to 10 Hz, and if you would play a movie at that refresh rate, you would most likely realize the gaps between each of the movie’s frames. For this reason, this value should not really be considered in a generic enough context.
The human eye’s sensitivity is different for different conditions and portions of an image, so you should be careful and consider higher refresh rates as necessary, to accommodate this.
This link has further information about how the screen characteristics and their changes are perceived by the human eye, and may give you an idea of which refresh rates you should aim for in a given context, based on the visual impact of your script.