@Carl I have the same issue as above. My wireless speeds from the main unit has been fluctuating around 300Mbps only though I have a 1Gbps service. Deco app test by hardwire reads around 900Mbps. Why does my wifi speed show less than 50% of the theoretical of even the 5G speed of the deco? I would be happy with 500Mbps. 
 

I have done the switching of deco units, switching of ethernet ports on the deco unit, even reset to factory defaults. Results all came back the same around 300Mbps. 

@genengjy 

What is the device you are connecting with?  Is it a seperate network card and if so what is the antenna array?  The speed concern may be a result of your device network adapter so before we lump it in with other user concerns I would like to rule that out.  

@CramNevets Brilliant. Simply brilliant.

@CramNevets  Thank you! My speed was capped by Qos to 50%. Now my speed is back to 590/40 Mbps wired. The speed through the wifi-connected deco's is not that great, but I can make Teams-calls again. 

@CramNevets I created an account just to say thank you! Finally getting more than what my initial speed test from a few years ago limit to. Now to google it up and see if it's worth upgrading to an M9 lol. Thanks again!