I changed one router setting, and the lag during big downloads just disappeared
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Lag during online gaming caused by simultaneous downloads is a congestion problem, not a bandwidth problem. Basic router QoS can prioritize devices (e.g., putting your gaming PC at highest priority), which fixes lag when other devices are the bandwidth hogs. For older routers without adaptive QoS, flashing OpenWrt and enabling SQM (Smart Queue Management) solves the deeper issue of bufferbloat by scheduling individual traffic flows fairly. SQM is CPU-intensive and may reduce throughput on older hardware — switching from CAKE to fq_codel helps. For most users with modern routers, enabling adaptive QoS is sufficient without needing OpenWrt.
