Interaction design laws
Doherty threshold
People stay engaged and productive when a system responds in under about 400 milliseconds, so neither the person nor the computer waits on the other.
Below about 400 milliseconds, an interaction feels like a conversation. Above it, attention drifts and people start to wait instead of work.
Respond instantly where you can. Where you can't, show something at once: a skeleton screen, an optimistic update, or progress, so the system feels responsive while the work finishes.
In practice
A to-do app that shows a ticked task immediately and saves it in the background, instead of waiting for the server.
Common mistake
Animations so slow they become the wait. A transition that takes a second makes every click feel sluggish.
Where it comes from
From Walter Doherty and Ahrvind Thadani's 1982 IBM paper, The Economic Value of Rapid Response Time, which found productivity rose sharply when computers responded in under 400 milliseconds.