So I just got the deco M9 plus system yesterday because I was having coverage issues at my home and it seems to have fixed that which is great but I noticed something odd.
My Internet package is 250mbps download and upload and the maximum bandwidth I can get on the deco is always below 100mbps, on both wired and wireless. I tested this standing very close to primary deco and measured speed on phone and laptop on WiFi, also tried plugging in my laptop directly to the primary deco and also wirelessly in other locations in my home. When I disconnected the deco and plugged in my old router, I was getting the expected speeds (over 200mbps).
I have already set the total bandwidth in the QoS to 1024, with no high priority devices and set it on standard mode.
I tried changing the ethernet port on the deco that is connected to the modem, and the speeds went up to over 200mbps for maybe 15-20 minutes but when I checked again later, they were back down to below 100mbps.
Firmware is updated to the latest version, primary deco is connected directly to ISPs modem and secondary deco is quite close to the primary deco, no walls between them. All my speedtests have been with the speedtest built into the deco app as well as speedtest website/app.