In Chapter 1, Faster Web—Getting Started, of this book, we mentioned that performance is also about perception. Indeed, as previously stated, time measurement depends on the moment of measurement and can vary depending on the complexity of the task to be performed, the psychological state of the user and the user's expectations as he might have defined them according to what he considers to be the software of reference when executing a certain task. Therefore, a good manner in which an application does what it has to do also means that the software would have to meet the user's expectations as to how this computer program ought to do things. Hence, quicker is not always better.

In this chapter, we will try to better understand the principles behind UI design when it comes to the perception of performance. We will see how these design principles can have a real effect on the user's subjective perception of time and improve perceived performance when there...