Some HCI researchers say that it is not so simple: users "attune" to a particular system's response rate regardless of its duration.  Ritchie and Roast say that user satisfaction with web performance is more complex than simple numeric response times. Users form a mental model of systems they are dealing with based on system response characteristics. To form this model, users perform a "selection and adjustment [of] subjective time bases, and adapting the rate at which the environment is monitored to meet its particular pace."  Attuning is the process of forming this mental model and adapting our expectations to a particular system's response rate.
Consistent response times and adequate feedback help users attune to a system's pace. Inconsistent response times and poor feedback reduce the "attunability" of a particular system, and "temporal interaction errors" ensue. Thus "the less variable the duration of a particular task, the more likely that users can attune to the environment"  and the more accurately users can distinguish tasks of differing duration.
Humans can attune to a remarkably varied range of response rates, anything from years to seconds. Everyone knows that postal mail takes a matter of days, that Domino's delivers pizza within minutes, and that traffic lights change in a matter of seconds. The web is different, however.
On large decentralized networks like the web, the effects of latency can exceed the effects of improvements in performance. Conventional performance engineering and evaluation are not possible in this environment.
Chaotic large-scale systems like the web can introduce non-deterministic delays. An external object in a web page can take anywhere from tens of milliseconds to what seems like an eternity (30 to 40 seconds or longer) to download. Rigid performance metrics such as response times under 10 seconds can be less important than consistent response rates.
To meet the needs of users, you need to provide an environment with characteristics to which they can attune. Consistency of response times and feedback allows users to better "attune" to system delays.
The key to attunability is to minimize the variability of delays. Variability is the difference between the slowest and fastest possible response rates. "The larger this variation, the less well system delays can be associated with a task," and the lower the system's attunability.  By minimizing this range, you allow users to model your system more easily and adjust their performance expectations.
Designing for attuning implies the adoption of transparency as an architectural principle.  By offering feedback mechanisms as pages and objects download, you can ensure that users will minimize "temporal interaction errors" associated with inconsistent response times.
The idea is to offer feedback that matches user expectations. Linear progress bars, which match user expectations, can be used to give users real-time feedback. Server load, cache state, and file sizes can be displayed with server-side includes. All of these performance cues are designed to let the user know how the system is performing and form a mental model. Here is an example file size cue using SSI:
<a href="thisfile.zip">download this file</a> (<!--#config sizefmt="abbrev" --> <!--#fsize file="thisfile.zip" -->)
This code automatically displays the file size of the referenced file so the user can gauge how long it will take.
The antithesis of this concept is the Windows file copy animation. The system portrays the activity as an animation of pages flying across at a constant rate, independent of the actual progress being made. This is like a spinning watch cursor, which has no relation to the progress bar. The non-linear progress bar stalls near the end of the scale, while pages keep flying (see Figure 1.4). A better solution would be to create a linear progress bar, and change the animation to filling up a page or removing it entirely.
Users "attune" to the speed of the web's response. If your pages are slower than average or are inconsistent in size, users tend to tune out and go elsewhere. Optimizing the size of your pages and making them respond consistently can help users establish a rhythm as they surf through your site. Throw in a compelling experience, and some sites can attain the most elusive of web site goals, flow. You'll learn more about flow in Chapter 2, "Flow in Web Design."