1---
2title: Respond Within 400ms
3impact: CRITICAL
4tags: ux, doherty, latency, response-time
5---
6 
7## Respond Within 400ms
8 
9Interactions must respond within 400ms to feel instant. Above this threshold, users notice delay.
10 
11**Incorrect (no feedback during loading):**
12 
13```tsx
14async function handleClick() {
15  const data = await fetchData();
16  setResult(data);
17}
18```
19 
20**Correct (immediate optimistic feedback):**
21 
22```tsx
23async function handleClick() {
24  setResult(optimisticData);
25  const data = await fetchData();
26  setResult(data);
27}
28```
29 
30Reference: Doherty, W. J. (1979). Managing VM/CMS systems for user effectiveness.
31