@ClaudioT I had the same issue and it resolved on my desktop by updating the wifi drivers.  

Exactly what I had to do (kill 2.4 network and just run 5). Set up 2.4 on my internet router for IoT bits.

I had the same symptoms as many people in this thread. For me it actually helped switching the main Deco unit for some reason. I reluctantly did that, as I did not see the logic, but I have had a stable 250/250 since I did.

I wonder why this happens. Could it be that the actual hardware is worn out? Might have helped to just insert the cable into the other port on the main Deco, but I did not try that. The funny thing is I could get 250/250 for a couple of minutes, or sometimes hours, by connecting the main Deco to another port on the modem, but then it dropped to 100/100 again (also stable...). 

@ClaudioT Me too same problem. i had the S4 3pack and those sucked, then i returned them to amazon and bought the M5 3 pack and now these suck. i already did a return on them even tho im waiting on my Wifi 6 Asus RT-AX82U  router to get here. when that shows up i will pack these M5's up and send em back too. i dont really need mesh i just wanted wifi out to my detached garage. with the wifi 6 range there will be no need for mesh (glorified repeaters).