For some reason, Deco starts to slow down to the 80-90Mbit speed after a while. It happens not only with clients which are connected via WiFi, my laptop connection speed slows down. While this problems exists, I tested with Open Speed Test which runs on my Nas which is connected to the other Deco Unit which is connnected to my main Deco via Ethernet backhoul. I just only plug out all the ethernet ports on the main deco (coming from the modem and my laptop). Then it is resolved. I don't know if this happens because of QoS. I could not find anywhere to disable it. Because I know this problem was exist on my previous Deco M4. After I disabled it, speed problem was gone. I have the same problem again. I am not sure this problem occured after I update the firmware or not. 

I have tested via iperf3 which runs on my laptop and the openwrt router, It shows the slow speed and normal speed which can be resolved by plugging out of the ethernet cables coming to this main unit. Please fix this or make an option to disable QoS :)