I have a Deco M5 system and am having problems downloading large software updates. In my case these are from Apple and normally result in an error saying that the internet connection has been lost which resets the whole download which is likely to be several GB.  I have noticed whilst using the internet intermittent page loading problems where the connections seems to momentarily break, and likewise on videocalls momentary hanging.
 
I think the problem could be related to these issues... I would have added a comment to the M5 one but the thread has been locked.
https://community.tp-link.com/en/home/forum/topic/596080
https://community.tp-link.com/us/home/forum/topic/640478

I tried turning off Fast Roaming and this resulted in a very slow connection (1/100 of what it should be) even after verification that my computer was connected to the correct M5.