When I have to load new information from my web server via ajax after a user action, how fast should the information be displayed for the user in order to be a good working UI for the user?
I've always been told that a delay of 100ms is generally not perceivable. An answer in a similar StackOverflow thread says:
The 100 ms threshold was established over 30 yrs ago. See:
Card, S. K., Robertson, G. G., and Mackinlay, J. D. (1991). The information visualizer: An information workspace. Proc. ACM CHI'91 Conf. (New Orleans, LA, 28 April-2 May), 181-188.
Miller, R. B. (1968). Response time in man-computer conversational transactions. Proc. AFIPS Fall Joint Computer Conference Vol. 33, 267-277.
Myers, B. A. (1985). The importance of percent-done progress indicators for computer-human interfaces. Proc. ACM CHI'85 Conf. (San Francisco, CA, 14-18 April), 11-17.
I guess this should follow the classic usability guidelines for response time.
0.1 second is about the limit for having the user feel that the system is reacting instantaneously, meaning that no special feedback is necessary except to display the result.
1.0 second is about the limit for the user's flow of thought to stay uninterrupted, even though the user will notice the delay. Normally, no special feedback is necessary during delays of more than 0.1 but less than 1.0 second, but the user does lose the feeling of operating directly on the data.
10 seconds is about the limit for keeping the user's attention focused on the dialogue. For longer delays, users will want to perform other tasks while waiting for the computer to finish, so they should be given feedback indicating when the computer expects to be done. Feedback during the delay is especially important if the response time is likely to be highly variable, since users will then not know what to expect.
If the request is synchronous, it should take no longer than a couple of seconds, or the user will wonder what is wrong as the UI freezes. If you can use asynchronous requests, show a "loading" screen and progressively update the UI as soon as data come in, a longer loading time is probably okay.