My Deco M9 network had been up and running for a few years now. For the last several months I have been experiencing a significant delay with all connected IoT devices. (Wemo lighting, Ring doorbell/cameras, Harmony hub, Sonos speakers)

As an example:

When I try to turn on my Harmony hub via the remote or app, 10-15 seconds goes by before the device receives the command and immediately processes it. 

With Ring, the delay does not detect people, or when the doorbell is rung, it doesn't actually ring for 10-15 seconds. 

With Sonos, sending music or commands to these devices also has a 10-15 second delay. As do commands sent to Wemo lighting and Honeywell thermostat.

I've been working my way through all of these manufactureres one by one to troubleshoot. The result is similar, none of these devices receive the command for 10-15 seconds after the command is issued by a wifi connected app or control unit. I can change the device operating the app, and the same situation is present. 

I'm now looking at the Deco's as the potential issue as all of these devices are connected to the deco wifi network. I have tested connecting devices to another network, and the issue goes away... I suspect a firmware update may have caused the issue? 

I have disabled UPNN and QOS. Deco's are in wireless router mode. 1GB (Cat6) connected to each other (slave to master direct) Cat6 connected from master to ISP modem.