Latency

Latency is the time a small message takes to reach a host and come back: the round-trip time, in milliseconds (ms). It’s what you feel as responsiveness: how fast a page starts loading, how natural a call sounds, whether a game keeps up. It isn’t the same as speed: a fast line can still have high latency.

What’s good

• Under 20 ms: excellent, typical for a server in your own country.
• 20 to 50 ms: good.
• 50 to 100 ms: fine for browsing and video, noticeable in fast games.
• Over 100 ms: calls start to feel delayed, and games lag.

Distance sets a floor: light in fibre takes about 1 ms for every 100 km there and back, so a server across the Atlantic can’t answer in much under 70 ms, however good the line. Wi-Fi adds a few milliseconds, more when it’s busy.

The median, not the average

One slow answer can drag an average far up. Netutilus judges latency by the median, the middle value of the probes, so a single hiccup doesn’t change the verdict. The spikes still show, in the chart and in the maximum.

When latency rises

• Only while the line is busy: that’s bufferbloat.
• Already at the router: the Wi-Fi. See Wi-Fi Analyzer.
• At one hop and every hop after it: a slow link at that point. See Traceroute and MTR.

Related

• Jitter, how much latency varies.
• Ping, to measure it.