I have gigabit fiber internet and the Deco M9 plus. The two satellite routers are connected via ethernet backhaul.  When I first installed the system last year I was able to get near gigabit speeds from a computer connected to one of the satellites via ethernet. When QOS  was added to the firmware I am lucky to get half of that. If I connected a computer directly to the ethernet of the main Deco I still am getting much slower speeds. If I take the ethernet from my fiber ONT and plug that into my laptop I get gigabit speeds. 

There should be a way to turn off the QOS feature for higher speed connections, it's deliberately slowing things down on my network now.