My setup is 2 Deco M9 Plus units, with Ethernet backhaul between them.

The base Deco is using PPPoE with a DSL modem. 

I have set my Smart TV and VOIP adapter flagged as "High Priority".  Both devices are hardwired on Ethernet using an unmanaged switch.

My son's gaming PC (also hardwired Ethernet, directly plugged into the second Deco M9 Plus's spare port)

QoS has the "Streaming" category prioritized.

The problem:  Whenever my son runs a download within Steam to install a game, the entire network grinds to a halt.  Everybody in the house starts yelling at the same time.  I check the app, and it's always the same -- a heavy download from the PC.   QoS seems to do nothing.  When this happens, all devices (wired and Wifi) are unable to use the network.

I have set the Total Bandwidth values artificially much lower than the true bandwidth of my DSL service, which seems to help a little, but it still happens.  Actual DSL speed is 25 Mbit down, 10 up.  Deco setting is 17 down, 8 up.    (yes, I know 25 is not a lot, but it worked just fine with the old Wifi / router with no hiccups, ever.)

Before the Deco M9 Plus,  I had an Apple Airport Extreme N (access point) with an old DD-WRT PPPoE router.   We *NEVER* had problems sharing the network, multiple people streaming HD, downloads, it all worked great.  The DD-WRT had many more granular QoS settings, which was very useful to fine-tune which devices get better priority.  So...

I would like to see the Deco M9 Plus add more levels to the QoS priorities. 

I would also like to see it do a better job of balancing the load when one device is saturating the network.

I would also like my family to stop yelling at me about this regrettable "upgrade" I did to the home network.

Thanks for your time..