I have a 10Gbps fiber connection wired to the main AXE200 deco, with the other doing wireless backhaul. Performance has always been from 200Mbps to 1Gbps+ on wireless. But for weeks, the wireless has been weak, video calls dropping. Testing the secondary AXE200, I was getting only 85Mbps with not great latency.

I was about to buy a third unit to solve the issue, and then just started playing around with settings. Turns out that the QoS (which only lets you specify a network up to 1Gbps) that I had set for two devices was ruining the entire network performance. Turned that off and devices connected to the remote AXE200 (the only two with priority on WoS) immediately jumped from 85Mbps down to 850Mbps down. That's the literal defintion of decimate - turning QoS on reduced the network performance by 90%.

I don't why QoS is that bad on the Decos, but in my case, turning it off restored the network to actually being usable again.