W3C HTTP 1991

Note: The cost of retrieval time

The assumption that the cost to the user associated with a certain retrieval time is linear in that time is wildly innaccurate. The real function could be very dependent on circumstances (like go to infinity at a deadline).

A better general approximation might be logarithmic for large time delays, and linear for small ones, like a*log(b*t-1) which has two parameters.