Speed is a key component of usability, which helps determine system acceptability.  How acceptable a system is determines its adoption rate. With over half of the IT projects deployed in the U.S. abandoned or underutilized ,  it is important to make systems and sites (many of which are big IT projects themselves ) that people actually use.
Part of our psyche, it seems, is devoted to understanding whether a particular system will have a big enough payoff to warrant the necessary expenditure of our time and energy. Brian Shackel characterized this paradigm as "system acceptability," which is a tradeoff between three dimensions:
Utility Or perceived usefulness. Is it functionally efficient?
Usability Or perceived ease of use. Can users work the system successfully?
Likability The user 's subjective attitude about using the system. Do users feel it is suitable?
All of these factors are weighed against each other and the cost of using the system (see Figure 1.1). Seen through Shackel's lens, when users make decisions about using a web site, they weigh how useful it will be, its perceived ease of use, its suitability to the task, and how much it will cost them both financially and socially . That's why sometimes we are willing to put up with difficult sites if the reward for doing so is large enough.
Traditionally, HCI research has focused on the quantification of Shackel's second dimension, usability. There is compelling evidence, however, that the utility of a technology should first be measured before any usability analysis occurs.  ,  If you can't accomplish a task, it doesn't matter how easy the system is to use. Likability, Shackel's third dimension of acceptability, is most closely associated with "flow,"  or emotional appeal .
The relative importance of usability changes over time. At first, usability has a strong effect on system use. As users gain more experience, they become more confident and believe they can accomplish more tasks with a desired level of performance (also known as self-efficacy  ). As a result, ease of use fades in importance and utility, and likability increase in relative importance. Usability then indirectly influences usage through utility (usability -> utility -> usage).
Designers tend to favor ease of use over utility. Davis found that utility has far more influence on usage than usability, however. "No amount of ease of can compensate for a system that does not perform a useful function." 
Speed plays a key role in all of these dimensions, especially usability and likability, so it is an important determinant of system acceptability and usage. In other words, how responsive your site is will in large part determine its adoption rate, which in turn affects your bottom line.