@WhyFi 

Go into the QoS settings to make sure the speeds configured there are not slower than your ISP speed. Also, undo any devices that were set for high priority.

Set QoS to standard, unplug the satellite unit just to make sure you aren't connecting through the remote Deco even though you are close to the main Deco.

If speeds are still slow on multiple devices, try swapping the main Deco with the satellite unit.

@WhyFi  I have 3 Deco P9 units and ATT Gigabit service as well.  I have had them for about a year now, and around 4 months after buying them I noticed that my speeds on Wifi and LAN were nowhere near what they should have been.  I unplugged the LAN cable from my gaming PC to the main Deco LAN and plugged it directly into my ATT gateway to see if my speeds were still slow with the Deco out of the equation.  I began pulling about 900mbps straight from the gateway so I knew that the problem was somehow related to my Deco setup.  I went through every setting in the app and couldn't find any reason why my speeds were being throttled by the Deco so I finally contacted TP-Link's tech support.  After troubleshooting for a while the woman I spoke with asked me if I had the Qos turned on.  I told her I had turned it on a few weeks before this happened and she told me to try turning it off completely and removing any devices I had added to it.  After doing so, I rebooted the Deco's through the app and sure enough, when they booted back up and I checked my speeds I was once again pulling 800-900mbps on my LAN devices and 300-400mbps on my Wifi devices.  

I just reread your original post one more time and I just want to note that even with Gigabit service myself, I have never pulled more than 500mbps on any Wifi devices in my home.  That includes before I ever got the Deco units that I have, when I was running directly off of my ATT gateway.  I don't know if you were pulling faster speeds than that before you got your M9 units or not, and I do not know what the M9 specs are as I have the P9's.  If you were pulling faster speeds before you hooked up your M9 units then I would try disabling the Qos altogether, rebooting and trying again.  I will say that speed tests can differ greatly as they depend on many factors such as what server each test is connecting to.  I have also found that the built-in speed tests on any network devices I have ever owned always claim faster speeds than a traditional speed test will.  I'm sure I don't need to elaborate on why that may be :).

The last advice I can add is that even if your Wifi connected devices are pulling 300-400mbps speeds, that is still fast enough to do almost any of the most bandwidth intensive activities you will likely do on any Wifi device in your home.  Even streaming 4K vids, as I have never had an issue in our home on any TV and I have 3 4K TV's hooked up to my Deco system's Wifi.  I have found that no matter what speeds any network devices claim they will achieve over Wifi, in practice I have never achieved them in real-time.  There are so many different factors that will effect the speeds you get on Wifi connected devices, such as each device's capabilities themselves, how many devices are currently connected, distance, and how much total bandwidth all connected devices are using.  If you are already aware of anything I have stated please don't take offense.  I am just offering my personal experiences in the chance they may be of use to you.  

Good luck with your speeds!