Internet lags when someone else is downloading? That's bufferbloat (and how to fix it)
Ping is a steady 15 ms until someone in the house starts a game download, a cloud photo backup or a big upload. Then it jumps to 300 ms, calls freeze and the game rubber-bands. The line is not too slow. The queue is too long.
Why a full line makes everything slow
Your internet line is like a single checkout. A download sends data far faster than the line can pass it on, so the router and modem keep a queue. A small queue is healthy; a huge one means every new packet, including the tiny ones a game or call sends, waits behind seconds of download data. Wirebook's word page draws it: What is bufferbloat?
Uploads are usually worse than downloads, because upload speed is often a tenth of download speed on cable and copper lines, so a phone backing up photos or a laptop syncing a folder fills it instantly. If lag starts when someone gets home and their phone hits Wi-Fi, an automatic photo backup is a good suspect.
Test it in two minutes
- Open a terminal and start a continuous ping:
ping -t 1.1.1.1on Windows,ping 1.1.1.1on Mac or Linux. Note the normal time. - Start something that fills the line: a speed test, a large game download, or uploading a big video to a cloud drive.
- Watch the ping. A rise of a few milliseconds is fine. A rise of 50 to several hundred milliseconds is bufferbloat. Test upload and download separately.
Some speed tests show this directly as latency under load, loaded latency or a bufferbloat grade. The idle ping figure most tests lead with hides the problem.
Fix it properly: smart queue management
- What it does: the router deliberately sends slightly slower than your line, so the queue stays in the router, where it can be managed, instead of in the modem or at your provider. It then interleaves the flows fairly, so small game and call packets skip past bulk downloads.
- Names to look for: SQM, Smart Queue Management, Adaptive QoS, fq_codel, CAKE. Many gaming and mesh routers have one of these; open-source firmware such as OpenWrt has SQM as a package.
- Settings: enter your real measured download and upload speeds and start at about 90% of each. If the ping still rises under load, lower it a little more; if it stays flat, try nudging it up.
- Old-style QoS that just marks "gaming" as high priority helps less than SQM, because the bloated queue can still be in the modem, which those rules never reach.
Workarounds if your router can't do it
- Cap downloads: Steam (Settings → Downloads → limit bandwidth), consoles and many launchers can limit download speed or pause while a game is running.
- Schedule backups: set cloud photo and PC backups to run overnight or only while charging.
- Use a cable for the gaming device: this doesn't cure bufferbloat on the line, but it removes Wi-Fi's own queues and retries from the problem.
- If it happens even when nothing else is running, it is more likely Wi-Fi or packet loss: see good ping, jitter and packet loss.
Rough maths for your SQM numbers, such as 90% of 36 Mbps, is quick in the percentage calculator.
Related
Slow every evening (peak-time congestion) · Input lag calculator · How long to upload a video · Why videos start blurry, then sharpen · Video keeps buffering · Wi-Fi calls breaking up · Play it: the one-checkout shop (Wirebook) · How much upload do video calls need? · Wi-Fi extender vs mesh vs powerline · What is a good ping, jitter and packet loss? · Mbps vs MB/s: internet speed explained · Mesh Wi-Fi backhaul and placement · Wi-Fi keeps disconnecting
Questions
Why does my internet lag when someone else is downloading?
The download fills your line and the router or modem builds a long queue of waiting data. Game and call packets wait in that queue, so ping jumps. This is called bufferbloat.
How do I test for bufferbloat?
Run a continuous ping and start a big download or speed test at the same time. If ping rises by more than about 30 to 50 ms, you have bufferbloat. Some speed tests report it as latency under load.
What is SQM on a router?
Smart queue management. The router shapes traffic slightly below your line speed and shares it fairly between flows using algorithms like fq_codel or CAKE, which keeps ping low even while the line is full.
Will a faster internet plan fix lag when downloading?
It can make it happen less often, but a big enough download fills any line. Smart queue management fixes the cause.
Why do uploads cause more lag than downloads?
Upload speed is often much lower than download speed, so a phone backup or file sync fills it quickly and the queue grows.
Published · updated .