@Redstapler 

Moving the M9s to a different location can help speeds. The client device may not be connecting over to the closes node so using a wired computer to test the LAN speeds would help.

It would be best to first test the main router to see what the main Deco is working with or capable of getting.

Another thing to check is the QoS is that the speed setting is not lowered than your internet speed or else it will create an unintentional bottleneck.

@Redstapler Just purchased a set of 3 M9 plus.  In the same large room with the unit that acts as the main router I routinely get between 300 and 400 Mb per second. In neighboring rooms on the same floor I get between 200 and 300 Mb per second.  I get similar speeds with the two satellite units in the basement and on the third floor.  But my satellite units are wired with ethernet cable.  I have not tried using the satellite on Wi-Fi only.  I also purchased at the same time a three pack of Orbi AX4200 which are Wi-Fi six capable.  Setting the Orbis  up in the same rooms there is no doubt they were faster. I would routinely gets spreds between 400 and 500 Mb per second.  I ran each set up for about a week.  It seemed like I had some drops from the Internet with the Orbis and not the TP links. The TP link seems to transition from one unit to another better than the Orbi.  The three pack of the Deco M9 plus Was $199 at Costco.  The three pack of the Orbi AX 4200 Was $499. Based on my experience and price I kept the TP links.  I could buy three more of the deco units and have more coverage and still be to the good $100.