The complaint was always the same. Someone's video call would freeze, the iPad would buffer, the music would skip, and it would all happen at once. The internet "went down". Except it hadn't. The download speed was fine. The problem was the upload, and more precisely, what a saturated upload does to everything sharing the link.
This is bufferbloat, and once you've seen it you can't unsee it. A single backup pushing a few megabits upstream fills the modem's oversized buffer, and now every other packet, the ACKs for downloads, the control traffic for a call, the DNS lookups, sits in a queue behind a wall of backup data. Latency under load goes from 15ms to 600ms. The link isn't full. The buffer is.
measuring before touching anything
I ran the DSLReports speed test, the one that grades for bufferbloat, before I changed a thing. Idle latency was lovely. Under load it earned a flat F. That F is the whole reason to do this.
The fix lives in OpenWrt's SQM package, which wraps cake, the queue discipline that came out of the bufferbloat project. The idea is counterintuitive: you deliberately set your shaper below your real line rate, so the bottleneck moves from the modem's dumb buffer to your router's clever one. Your router can drop and reorder packets sensibly. The modem just hoards them.
the numbers that matter
My line syncs at roughly 75 down and 20 up. I set the shaper a touch under each, which is the part people resist because it looks like throwing speed away:
config queue
option interface 'eth0.2'
option download '72000'
option upload '19000'
option qdisc 'cake'
option script 'piece_of_cake.qos'
option linklayer 'ethernet'
option overhead '44'
The overhead value matters more than the percentages. For VDSL over PPPoE you're carrying ATM-era framing tax, and if you get it wrong the shaper is shaping the wrong amount. Forty-four bytes was right for my setup. Yours may differ; the OpenWrt wiki has the table.
I rebooted, re-ran the test, and the under-load latency held at around 25ms with cake doing its job. Same F became an A. I'd "lost" maybe four percent of throughput I never noticed and gained a house that stops complaining when the backup runs.
the verdict
This is the rare networking change you can feel. Calls hold up whilst someone uploads holiday photos. The telly stops buffering when a phone backs up in the background. Nobody in the house knows what cake is, and nobody needs to. They just stopped telling me the internet was broken, which is the highest praise home networking ever receives.