Latency and jitter at home, explained simply

Speed gets all the attention: the number in megabits on the box and in every test. But when a video call stutters or a game lags on a fast connection, the culprit is almost always one of two other numbers: latency and jitter.

Latency: how long an answer takes

Latency, or response time, is the time a small message takes to go somewhere and come back, measured in milliseconds.

• To your router, over good Wi-Fi, it's a few milliseconds.
• To a server on the internet, it adds the distance and the provider's network: often somewhere between ten and a few tens of milliseconds, more for servers on another continent.

Low latency makes things feel instant: a click answered immediately, a voice that doesn't lag behind the lips.

Jitter: how steady that time is

Jitter is how much the response time varies from one message to the next. A connection that answers in 10 ms, then 11, then 9 has low jitter. One that answers in 10, then 80, then 15, then 200 has high jitter, even if its average looks fine.

Real-time apps hate jitter more than slow-but-steady latency. A call can adapt to a constant delay; it can't adapt to one that keeps jumping, so it freezes, drops words or speeds up the audio to catch up.

How Wi-Fi makes both worse

Inside the home, Wi-Fi is often where latency and jitter are born:

• A weak link means lost transmissions that have to be repeated. Each repeat adds delay, and repeats come and go: that's jitter.
• Interference and crowding make devices wait for a free moment to transmit. See neighbors' Wi-Fi interference.
• An extender or node with a weak link to the router adds its own delays on top.
• A busy line. When someone uploads or downloads something large, the queue in the router can fill up and push everyone else's response time up. Some routers have a setting (often called QoS or smart queue) that keeps that queue short.

That's why the same connection can show low latency next to the router and ugly jitter in the far bedroom.

What to look at

Speed tests often show latency once, at the start, on a quiet connection. More useful is watching response time over a minute or two in the room you care about: how low it is, how much it jumps, and whether some requests get no answer at all. Those three together explain what you feel on video calls and in online games.

Measure it room by room

WiFi Heatmap measures the response time to your router, how much it varies and how many requests go unanswered, in every room, and turns them into a map. The rooms where jitter climbs are the ones where calls and games will struggle, whatever the speed test says. See also full bars but slow Wi-Fi.