Yahoo! engineer Stoyan Stefanov helps fellow developers understand the importance of "relative time" with a review of experiments that show how emotional state exaggerates the differences between actual, expected, perceived and remembered time.  Time slows when the experience is unpleasant, unknown, boring or overwhelming in detail, and speeds up when tasks are successful and parameters are understood.  He spends some time describing a time perception scale developed by Microsoft researcher Steven C. Seow, PhD, which ranges from "instantaneous" at 0.2 seconds to "captivity" at 7-10 seconds. 

Stefanov introduces psychologist Mihaly Csikszentmihalyi's concept of "flow," which is an optimal state of mind where the person is fully immersed in what he or she is doing.  Applying this idea to web sites, Stefanov suggests that it is possible to keep users "in the flow" by using certain techniques to help them perceive that their Internet explorations are useful, their time is well-spent, and the Internet processes are reasonable and predictable.

Stefanov goes on to detail some of these techniques, such as how and when to use progress indicators, minimizing the negative effects of waits by emphasizing benefits and eliminating uncertainty through wording that prevents a feeling of imposition on the users' time.  He also reminds developers to test their intuitions, making sure to understand that users sometimes have surprising expectations.