@cgarry I assume that the Deco nodes are not connected by Ethernet cables, is that correct?  

What you observe is, roughly, expected.  Wifi works like a radio transmission and has a limited capacity.  

Traffic sent to Internet by a device connected to the garage Deco is transmitted three times over the wifi network.

Do the Deco use also the 2.4 band to communicate among them?

  @yves_b you are correct they are connected over WiFi not ethernet. No, I don't use the 2.4GHz channel, a previous forumn mentioned that this could cause some latency so I switched completely to 5GHz. Is there any benifit in using both?

  @cgarry The backhaul should automatically use all available bands.  The app shows, in the details of each satellite Deco, the bands actually used.