Users really care about speed in interaction design...A snappy user experience beats a glamorous one, for the simple reason that people engage more with a site when they can move freely and focus on the content instead of on their endless wait.
0.1 seconds gives the feeling of instantaneous response. This level of responsiveness is essential to support the feeling of direct manipulation.
1 second keeps the user's flow of thought seamless.
10 seconds keeps the user's attention. A 10-second delay will often make users leave a site immediately.
Engineers seem to have a clear, if usually implicit, model of the process of design. It is usually an orderly model of an orderly process as the engineer conceives it.
The notion that the design process should be modeled as a systematic step-by-step process seems to have first developed in the German mechanical engineering community.
Herbert Simon independently argues for design as a search process in The Sciences of the Artificial. He was motivated to lay out a strictly rational model of design precisely because such a model was a necessary precursor to automating design. His model remains influential even if today we recognize the "wicked problem" of original design as one of the least promising candidates for AI.
In software engineering, Winston Royce independently introduced a seven-step Waterfall Model to bring order to the process. In fact, Royce introduced his waterfall as a straw man that he then argued against, but many people have cited and followed the straw man rather than his more sophisticated models. Even if ironically, Royce's seven-step model must be considered one of the foundational statements of the Rational Model of Design.