Bufferbloat Test
Bufferbloat is why a fast connection can still feel slow. This test measures latency while the line is deliberately saturated, in both directions, and grades the increase.
Bufferbloat test
Idle latency, then latency while the line is saturated in each direction.
Idle latency
—ms
Loaded download
—ms
Loaded upload
—ms
Added (download)
—
Added (upload)
—
Data used
—
Ready. Nothing is measured until you start.
What bufferbloat actually is
Network equipment queues packets when the link is busy. A small queue absorbs bursts; a bloated one holds packets for hundreds of milliseconds because its buffer is far larger than the link needs. The classic symptom: a video call is perfect, someone starts a cloud backup, and the call breaks up completely even though the speed test still reports hundreds of megabits.
The fix is usually a modern queue-management algorithm — fq_codel, cake or your router's Smart Queue / SQM feature — not more bandwidth.
How the grade is calculated
The test measures idle latency, then saturates the download path with real transfers while sampling latency, then does the same for the upload path. The grade comes from the largest increase over idle:
| Added latency | Grade | What it means in practice |
|---|---|---|
| Under 5 ms | A | Queue management is working well. |
| 5–30 ms | A | Barely noticeable during calls and games. |
| 30–60 ms | B | Occasional stutter during large transfers. |
| 60–100 ms | C | Clear lag whenever the line is busy. |
| 100–200 ms | D | Heavy bufferbloat; real-time traffic suffers badly. |
| Over 200 ms | F | The connection is unusable for calls during transfers. |
What to do about a bad grade
Start with the router. Look for settings named SQM, Smart Queue, QoS or "Gaming preferred / anti-bufferbloat", enable them and set the uplink and downlink limits to about 90% of your measured speed. If your router has none of these, a firmware such as OpenWrt adds them. Re-run this test after each change — the grade should move.
If the grade stays bad on a wired connection with queue management enabled, the queue may be upstream in the ISP network. That is worth raising with your provider, with this page's results as evidence.