@TWPS 

@TP-Link answers inline below. To note, this setup has been in place for over 2 years and just recently (after the last firmware upgrade) did all of these issues start. If I connect all of these devices to the ISP's wifi router directly there is no connectivity delay. 

1. Can I have a picture of the network topology of your Deco system?

Fibre ISP router -> Deco M9(1) -> Deco M9(2) Directly connected with Cat6 cables 

Directly connected to Deco M9(2) is a Windows 10 PC

Deco M9(1) main floor, Deco M9(2) second level, ISP router basement

2. Did Deco work on the access point or wireless router mode?

Router mode (required as I use parental controls - reason for purchase)

3. Did turning off the fast roaming as well as the beamforming on the Deco make any difference?

I do not use fast roaming as some devices on my network do not work with this technology. Beamforming is enabled, disbaling does not cause any changes. 

4. If possible, would turning off the 5ghz on the Deco help a little bit;

This does not make a difference

5. Please also turn off the Mesh technology of the smart home devices on the Deco APP;

Mesh technology is not used at all

@TP-Link 

I'm pretty sure this is a channel issue. The Deco M9's are using the most congested channels in my neighbourhood. I ran network optimization from the deco app and used a wifi channel scanner. There are many empty channels, but the most congested are picked up and used by the deco devices. 

Aren't these devices supposed to pick the least used channel???

Some of my devices are reporting the channel is saturated and quality is fair. They also recommend a different wifi channel be used. 
 

How do I change the wifi channel? I want to use the channels not in use and saturated. 
 

The ISP router uses channel that are not in use by others and I suspect this is why devices work well.