The human behavior surfing a website?
Here's what Jakob Nielsen wrote about. It's pretty interesting and it means that a website responsiveness should be under the second.

• 0.l seconds gives the feeling of instantaneous response — that is, the outcome feels like it was caused by the user, not the computer.


• 1 second keeps the user's flow of thought seamless. Users can sense a delay, and thus know the computer is generating the outcome, but they still feel in control of the overall experience and that they're moving freely rather than waiting on the computer.


• 10 seconds keeps the user's attention. From 1–10 seconds, users definitely feel at the mercy of the computer and wish it was faster, but they can handle it. After 10 seconds, they start thinking about other things, making it harder to get their brains back on track once the computer finally does respond.

Source: http://www.useit.com/alertbox/response-times.html