@tonymcgra,

Something I have started to see a few users do, is to set the QoS settings above what is expected from your ISP by increasing the bandwidth gradually, some people were able to see increases in their overall speed; although I expect this to be an unintended behavior.

Some things to check:

Is your modem in Bridge Mode? Or Providing a Wireless Network?

Disable UPnP on Deco.

What type of Internet did you select during setup? Dynamic IP, PPoE, etc 

What does your network setup look like? Is there a switch between the modem and router?

  @tonymcgra,

Are the speeds similar when you are connecting your laptop to the router via an Ethernet Connection;

The first step that is recommended for troubleshooting this case, is to replace the Ethernet cable between the router and modem. (You would be surprised at how often this is the case).

Otherwise, I am finding that this is the setting that you should adjust. The QoS Feature may be limited your bandwidth severely for some reason. There is also a "get internet speed" test available in this menu. Also, make sure that there are no devices with high priority to see if this is causing the issue.

Using QoS feature of Deco

  @tonymcgra,

If you remove the satellite Deco node from the configuration, does the speed stay the same? I am wondering if the signal has started to degrade to such an extent that the wifi given by your powerline satellite deco, may be affecting the overall quality of the mesh network, you could try to move the satellite deco into the same room, to ensure that it is on the same circuit. 

In the deco app, you can also manually configure what backhauls are used by the individual decos, I would maybe recommend messing around with changing the decos to use a wireless backhaul, to see if the wiring in your walls is a potential cause of the slowed speeds.

I am racking my brain trying to come up with a reason why a network would have great performance and then degrade over an hour period into almost nothing, maybe the wifi settings on your Decos need to be changed to better allow devices to reconnect to the network, perhaps by changing the encryption settings for your mesh network.