Fast line, still buffering

A speed test reports the biggest number your line can produce for a few seconds with nothing else running. Streaming asks for something different: a steady flow, packet after packet, while the rest of the house is also online. A connection can be excellent at the first and poor at the second.

Throughput is not the thing that breaks

Video needs a fraction of a modern connection — a few megabits for high definition, rather more for 4K. If a test says 200 Mbps and playback still stalls, the missing capacity was never the problem. What stalls a stream is delay that varies: packets arriving in bursts and gaps rather than evenly, so the player empties its buffer faster than it refills.

The measurements that describe this are latency under load, jitter and packet loss. Several speed tests now report all three; the ones that show only a download figure cannot tell you anything about it.

Bufferbloat, the usual culprit

When a link is saturated, equipment along the path queues packets instead of dropping them. The queue grows, and with it the delay for everything else, which is why a large upload or a game update can make video stutter on a line that tested fine minutes earlier. Latency that sits at 20 milliseconds idle and climbs to several hundred under load is the signature.

The fix is on your side of the line. Modern routers offer smart queue management, sometimes labelled SQM or simply "bufferbloat", which keeps those queues short. Turning it on costs a little peak throughput and removes most of the stalling.

Isolate it in three tests

Run one test on a cable straight into the router, one on Wi-Fi in the room where the buffering happens, and one while somebody is deliberately loading the line. If the cable is clean and Wi-Fi is not, the problem is the wireless path: interference, distance or an older client. If both are clean until the line is loaded, it is bufferbloat. If all three are clean and only one service buffers, the congestion is somewhere between your provider and that service, and neither a new router nor a faster plan will move it.

Repeat the same test at a quiet hour and at nine in the evening. The comparison separates a fault on your line from the daily rush everybody on the street shares.