Bufferbloat

Bufferbloat is the extra delay a packet waits in a full queue. When a download, an upload or a backup fills the line, the router holds the packets it cannot send yet, and every game packet and call packet waits behind them. A connection can ping well when idle and lag badly the moment someone else uses it, which is why an idle ping alone never shows it.

SystemCheck's bufferbloat test measures it directly. It times small round trips on an idle line, then one every 200 ms while the line downloads and while it uploads, ignoring the first 1 s of each phase while the connection ramps up. The increase over idle in the worse direction sets the grade, on Waveform's bands: A+ under 5 ms added, A under 30, B under 60, C under 200, D under 400, F at 400 or more. Read the milliseconds added before the letter, since two results a grade apart can feel the same.

Upload is where it usually shows, because the upstream is the narrow side of most home lines. The fix is active queue management on the router, often listed as Smart Queue Management or QoS, with algorithms such as fq_codel or CAKE; RFC 8290, which specifies FQ-CoDel, describes it as a tool for fighting bufferbloat. A faster plan does not cure it on its own: a deep queue still adds delay once it fills.