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Most people look only at the download figure. The other three are usually what you actually feel.
How fast data reaches you. This is the number plans are sold on, and the one that matters for streaming, browsing and downloads.
How fast data leaves you. It matters for video calls, cloud backup and sending large files. On wireless networks it is always well below the download figure.
The round trip time for a single packet, in milliseconds. Under about 50 ms feels instant. Latency, not speed, is what decides whether a video call feels natural.
How much the latency wobbles. Steady latency beats low-but-erratic latency — jitter is usually the culprit when a call breaks up while the speed test looks fine.
Real speeds depend on signal strength, how busy the local cell is and the time of day. These are the figures published in each plan’s Broadband Facts.
Wi-Fi is usually the limit, not the line. If the number beside the gateway is much higher than the number in the far room, that is a coverage problem — placement guide.
Nine times in ten it is one of the first two.