Textbook of Usability

The Doherty Threshold states that productivity soars when a computer responds to user input in under 400 milliseconds. Walter Doherty and Arvind Thadani of IBM established the threshold in a 1982 paper showing that below 400 ms, users enter a state of engaged flow; above it, they become impatient and disengaged.

Their research on IBM mainframe users found that reducing system response time from 1 second to sub-second had outsized effects: user think time decreased, productivity increased, and user satisfaction improved dramatically. The magic was in the cognitive state maintained by fast response.

The threshold connects to perceptual and cognitive architecture:

• 100 ms — the Model Human Processor's perceptual cycle time. Delays under 100 ms feel instantaneous.
• 400 ms — the Doherty Threshold. Below this, the user maintains their train of thought.
• 1000 ms — the classical "user flow" boundary. Beyond this, users consciously wait.
• 10 seconds — the typical attention limit. Beyond this, most users abandon the task.

Applied to modern software:

• Page loads should complete within 1 second to feel fast
• Interactive feedback (hover, click response) should be under 100 ms
• Complex operations should show progress indicators if they exceed 400 ms
• Long operations should provide both progress and estimated time
• Optimistic UI updates (showing the expected result before confirmation) can mask backend delay

Performance is a usability issue, not merely a technical one. Every millisecond of responsiveness affects the cognitive experience of the user.