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Fix lag under load (bufferbloat)

Why games stutter and calls freeze when someone else is downloading — and the router setting that fixes it.

When your connection is busy, your router or modem queues up far more data than it can send. Every packet, including your game or video call, waits at the back of that queue. That extra wait is bufferbloat, and it is why your ping jumps from 20 ms to 300 ms while a download runs.

The fix is Smart Queue Management (SQM): your router deliberately runs a little below your line speed so the queue stays in its control, and uses an algorithm (CAKE or fq_codel) that lets small, time-sensitive packets skip ahead.

Turn it on

To open your router's settings, try these addresses (one of them usually works): 192.168.0.1, 192.168.1.1, 10.0.0.1, 192.168.50.1. The login is often printed on a sticker on the router.

  • OpenWrt / GL.iNet: install luci-app-sqm, pick your WAN interface, choose cake with piece_of_cake.qos.
  • Ubiquiti UniFi: Settings → Internet → your WAN → enable Smart Queues.
  • eero: in the app, eero Labs → Optimize for Conferencing and Gaming.
  • ASUS: Adaptive QoS → QoS → enable, type Adaptive QoS or Traditional. ASUSWRT-Merlin offers fq_codel/CAKE.
  • Netgear Nighthawk: Advanced → Setup → QoS → enable Dynamic QoS.
  • pfSense / OPNsense: create FQ-CoDel limiters on the WAN for both directions.
  • ISP-supplied router with no QoS: put your own SQM-capable router behind it, and set the ISP box to bridge/modem mode if it supports that.

Set the speed limits

Start at about 90% of your measured download and upload, then re-test. If the grade is still B or worse, drop to 85%, then 80%. You give up a little peak speed for a connection that stays responsive. The aim is less than 15 to 25 ms of extra lag under load.

Your Smart Queue numbers

Enter the speeds your Lag Check measured, wired if you can.

Pick cake (or fq_codel) where the router offers a choice. Some routers ask for kbit/s, others for Mbps.

Enter both speeds to see your limits.

Low-latency networks (L4S)

Some providers (Comcast in parts of the US since 2025, T-Mobile) now run L4S, which keeps queues short for apps that support it, such as FaceTime, GeForce NOW and Steam. It helps only when the network, the app and the device all support it, so SQM is still the fix for everything else on your line.

Still bad?

If SQM doesn't help, the queue may be inside your modem or at your ISP. Upload direction is the usual culprit on cable and DSL. Show your ISP the Net Fix result; a line with an F grade under load is a legitimate fault report.

Made a change? Measure again to see the difference.

Run the Lag Check