Why your fast internet feels slow: bufferbloat and latency under load

Here is a complaint we hear constantly: "My internet is fast, but when my partner starts streaming, my Zoom call falls apart." The speed test shows plenty of bandwidth. Nothing seems broken. And yet.

The cause is bufferbloat: a router or modem holding too many packets in a queue, so that everything behind the queue waits (Bufferbloat.net). The symptom is latency that is fine when the line is idle and terrible when it is busy. That is exactly what the Latency under load number on Speedtesty measures.

The motorway analogy

Think of your connection as a motorway with a single toll booth. When traffic is light, you drive straight through — low ping. Now a convoy of lorries (a big download) arrives. A well-designed toll plaza lets small cars filter through between lorries. A badly designed one makes everyone queue in a single lane. Your tiny packets — a voice sample, a mouse click — sit behind megabytes of video and arrive late.

The lorries don't care; the download finishes just as quickly either way. But your call, which needs each packet now, suffers.

How we measure it

During the download and upload phases, Cloudflare's measurement engine keeps sending small ping requests every few hundred milliseconds. The median of those is your latency under load. We show the worse of the two directions, because the weakest link decides how your calls feel.

Then compare it to your idle ping:

Increase under load	What it means
Under 30 ms	Excellent. Your router manages queues well.
30–100 ms	Normal. You may notice calls softening when the line is busy.
100–300 ms	Bufferbloat. Expect glitches whenever someone downloads.
300 ms+	Severe. Calls and games will be unusable while the line is loaded.

A connection with 10 ms idle ping and 400 ms under load is, for interactive use, a 400 ms connection whenever anyone else is online.

How to fix it

The good news: bufferbloat is almost always fixable at home, without changing your plan.

1. Turn on Smart Queue Management (SQM) on your router. Look for settings called SQM, CAKE, fq_codel, QoS or Traffic Shaping. Set the limits to about 90–95% of your measured download and upload speeds. This is the single most effective fix.
2. Use a router that supports it. Many ISP-supplied boxes don't. Routers running OpenWrt, or consumer models that advertise "gaming QoS" or "anti-bufferbloat", do.
3. Check your modem's upload. Upload bufferbloat is the most common kind, because uploads on cable and DSL are small and saturate easily. If your upload is 10 Mbps, one cloud backup can clog it.
4. Re-test. Run Speedtesty again with SQM on. You will usually lose 5–10% of raw speed and gain a connection that stays responsive under load — a trade almost everyone should make.

Why most speed tests miss it

Because they measure latency before the download begins, when the line is idle. That is like judging a toll plaza at 3 am. Measuring under load is what makes the number useful — and it's why we put it on the main screen rather than hiding it in an "advanced" tab.