@Sav25 

Consistent 100% CPU utilization usually result of one of the following two things:

1. Hadrware failure of Main Deco unit. 

To test that, promote Satellite Deco to Main Deco and swap them: How to Set up the Satellite Deco or a new Deco to be the Main Deco. If problem goes away, you had Main Deco unit with hardware failure.

2. Main Deco can't handle the load.

Could be different reasons, because in Router mode Main Deco does a lot of things other than passing traffic. If your Main Deco is connected to ISP router/gateway, you can configure Deco mesh to run in Access Point mode: Switching a Deco to Access Point (AP) Mode

In that mode Main Deco does not do traffic management, such as antivirus or content filtering, it just passes the traffic. If CPU utilization drops and WiFi becomes stable, this is where the problem was.

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"I have also read customer support replies telling customers to turn off mesh features including Fast Roaming, Beamforming etc - I've tried this but no joy" - these recommendations you should ignore and turn these mesh features on.

  @Alexandre. Thanks for the advice, but either of those are bad outcomes (hardware failure or hardware not being able to keep up with the advertised features). Eero is ordered, so I'm done here anyway.

Maybe this post will help someone else with more time to burn on these things.

  @Sav25 hello my friend i ve Same problem with you. Do you solved it

  @Sav25 Having exactly the same problem.

Mesh wi-fi around the house, 4 Deco M4 units. All working perfectly for a few years till I upgraded to this exact firmware.

Direct wi-fi to the ISP router works fine, 300Mbps Download, 100Mbps with <20ms pings.

After the upgrade all my Deco M4s are turning red every minute or so, speed test rarely reaches 50Mbps and pings are through the roof, >200mps.

No reset worked. Need to downgrade immediately!

So, bro how's everything? no solution from tplink?

Haha i doubting to upgrade my firmware after seen this post. 

  @Sav25 

It turned out I was able to solve my problem, and it didn't require another change in the firmware.

My steps:

- Disconect all Decos from power;

- Delete all Decos from the app;

- Reset the ISP router. Turn off, turn back in;

- Opened the Deco App and setup everything from stratch, starting by powering on and adding the Decos one by one.

Ultimately I don't know what solved it, but these steps worked for me. It's a shame that one has to delete and power off everything and go from scratch, resets in the Deco App are useless and there is ZERO support from the company.
Is it too much to ask to have a section in the manual or in the App that suggests doing this?! FFS