Firefox products should look and feel fast!

Elements of Performance

The perception of performance is largely determined by four factors: Duration, Responsiveness, Fluency and Tolerance.


Duration is the actual duration that a process takes. This is the element that is often referred to as “performance” in technical discussions. Different magnitudes of duration require different treatments to achieve an optimal perceived performance.

Terminology Duration Response
Instantaneous up to 100ms Acknowledge user input within this time frame. Ideally, a visible process towards the completion of the task begins within this time span.
Immediate 500–1000ms Answers to simple requests must be completed within this time frame (e.g., opening a new window).
Continuous 2000–5000ms Answers to complex questions must be completed within this time frame (e.g., loading a web page).
Captive 7000–10000ms Users will begin switching tasks at this point. If a process takes longer than this, it should be segmented.

Why Performance Matters by Denys Mishunov Part 1, Part 2, Part 3.


Responsiveness is the perceived time it takes the system to respond to user input.

Example: An empty dialog appearing immediately after a click and then taking a second to populate with content feels faster than the same dialog appearing with a second delay, but fully populated.

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Fluency is the perceived smoothness of a process. It could also be described as a measure for how hard the machine appears to be working. Aim for constant 60 frames per second.

Example: A stuttering progress indicator gives the impression of lower performance, regardless of the actual duration of the process.

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Tolerance is a measure of how long the user expects a process to take and at what point they will abandon or cancel the process.

Example: The tolerated duration for loading a web page is much longer than for saving a bookmark.

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Bias and Priming

A user’s preconception about the speed of something – based on brand or prior knowledge – influences the perception of time.

Example: Users perceived a browser as being faster after reading an article about that browser having improved performance recently.

Other Influencing Factors

These factors can also influence the perception of performance:

  • Perceived task complexity
  • Emotional state
  • UI complexity and visual noise
  • Trends
  • Task frequency, attempts, failures

Literature to detail the concepts explained above.

Designing and Engineering Time: The Psychology of Time Perception in Software by Steven C. Seow Ph.D.

Thinking, fast and slow by Daniel Kahneman.

Designing with the mind in mind by Jeff Johnson.