API Latency: Why 200ms Feels Like Forever

Your API returns in 200 milliseconds. That's fast, right? A fifth of a second. Barely perceptible. Except it's not just the API. It's your server processing the request. The API call. Processing the response. Rendering the result. Now you're at 400ms. Add another API call because your page needs data from two sources. Now you're at 600ms. Your user tapped a button and nothing happened for more than half a second. They're already wondering if it's broken. The Human Perception of Speed Researchers have studied this for decades. The findings are consistent: Under 100ms: Feels instantaneous. Users perceive cause and effect as immediate. 100-300ms: Noticeable but acceptable. Users feel the system is working. 300-1000ms: Sluggish. Users notice waiting. Focus starts to break. Over 1 second: Disruptive. Users wonder if something went wrong. They consider clicking again. Over 10 seconds: Users leave. These thresholds aren't arbitrary. They're how human attention works. And your API latency is a