Respond Within 400ms

Interactions must respond within 400ms to feel instant. Above this threshold, users notice delay.

Incorrect (no feedback during loading):

async function handleClick() {
  const data = await fetchData();
  setResult(data);
}

Correct (immediate optimistic feedback):

async function handleClick() {
  setResult(optimisticData);
  const data = await fetchData();
  setResult(data);
}

Reference: Doherty, W. J. (1979). Managing VM/CMS systems for user effectiveness.