Bufferbloat: the fast-internet lag problem

What is actually happening

Every router and modem keeps a buffer — a waiting room for packets when the line is momentarily full. Buffers are healthy in small doses. But much home equipment ships with buffers hundreds of times too large.

When a big transfer fills the line, those oversized buffers fill too, and now every packet — including the tiny urgent ones for your call, your game, your clicks — waits in a queue behind megabytes of bulk data. Latency balloons from 20 ms to 300, 600, sometimes over 1000 ms. That is bufferbloat.

The telltale symptoms

The connection is fine until someone uploads photos, a game updates, or a cloud backup kicks in — then calls stutter, streams re-buffer, pages crawl and games lag. Stop the transfer and everything instantly recovers.

Because the trigger is your own traffic, bufferbloat hides from idle speed tests: download, upload and idle ping can all look perfect.

How to measure it

Measure latency while the line is saturated. Our test does exactly this — probes race the download and upload phases, and you get a loaded-latency figure for each plus an A–F bufferbloat grade. A grade of C or worse means real-time apps will suffer whenever the line is busy.

The fixes, in order of effectiveness

The real cure is Smart Queue Management on your router: algorithms like fq_codel or CAKE keep queues tiny by design. Look for settings named SQM, Smart Queues or QoS with fq_codel/CAKE; set the shaper to slightly below your measured speeds. Grades routinely jump from D to A.

If your router lacks SQM, options are: enable any built-in "anti-bufferbloat" or gaming QoS mode (varies from decent to placebo), replace the router with one that supports SQM (including anything running OpenWrt), or put the ISP box in bridge mode behind such a router.

What does not work: buying more Mbps. A bigger pipe with the same bloated buffers just fills faster.