Bufferbloat / Latency Under Load Test
Find out whether your connection falls apart when somebody else is downloading.
Run the test
Each sample is a real HTTP request to the nearest Cloudflare edge, with the response read in full. It is not an ICMP ping: it includes the server’s own handling time and stops at the edge rather than reaching any particular destination.
Where this comes from
Latency is measured while the connection is idle, then again while four parallel downloads are running, and the difference is the finding. That difference is bufferbloat: a router holding a queue of packets it should be dropping, so anything real-time waits behind a file transfer. It is why a video call collapses the moment somebody starts an upload, and it has almost nothing to do with how fast the line is — a gigabit connection with 400 ms of bloat is worse to live with than a 30 Mbps one with none. The throughput achieved during the loaded phase is reported alongside the grade, because a clean result on a link that was never actually busy proves nothing.
What the results mean
- Added by the load
- How much latency appeared when the connection got busy. Under 5 ms is excellent, under 30 ms is fine, past 200 ms the connection is unusable for calls while anything is downloading.
- Grade
- The same A to F bands the established bufferbloat tests use, so the result is comparable with them.
- Load achieved
- Throughput during the loaded phase. Below about 5 Mbps a clean result says more about the test than about the connection.
- SQM
- Smart Queue Management — the router feature that fixes this by keeping the queue deliberately short. Also called fq_codel or Smart Queue.
Common problems and fixes
- The result says inconclusive
- The test could not load the connection enough to fill a queue. That happens on fast lines; running it while something else is downloading gives a meaningful number.
- Calls break when I upload, and this says the connection is fine
- This tests the download direction. Upload bufferbloat is at least as common — and worse, since home connections have far less upload capacity for a queue to build in.
- My router has no SQM setting
- That is the usual limitation of ISP-supplied hardware, and the main reason people replace it. The capability is firmware rather than horsepower.
Frequently asked questions
What is bufferbloat?
A router with a large buffer that fills up rather than dropping packets. When a download saturates the link, everything else queues behind it — so a ping that was 15 ms becomes 400 ms, and a video call falls apart while a file is transferring. It is a queueing behaviour rather than a capacity problem, which is why more bandwidth does not fix it: a bigger pipe fills the same queue faster.
How do I fix bufferbloat?
Enable queue management on the router — look for SQM, Smart Queue or fq_codel in its settings. It works by keeping the queue deliberately short, at the cost of a few percent of peak throughput, which is a trade almost everyone should take. If the router has no such option, that is a firmware limitation and the reason people replace ISP-supplied hardware.
Why does my fast connection feel slow?
Frequently this. Speed tests measure throughput on an otherwise idle line and report a large number; bufferbloat shows up only when something else is using the connection, which is most of the time in a household. The two measurements are independent, and this is the one that matches the experience.
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