@antonyb12000 

You should expect betters speeds. There may be multiple reasons why you see 80Mbps. Let's start with simple one: in Deco app, there should be Test Internet Speed button on main screen. When you run that speed test, what speeds do you have there? 

  @Alexandre. Hi,

I've run the test and get 510mbps download and 522 mbps upload

  @antonyb12000 

That proves the wired link between Main Deco and modem/router is good. If there are issues with Ethernet cable between them, Main Deco may drop link speed to 100Mbps. With Deco M5 you are supposed to be alerted about that, but it is always good to double check.

The next step is to check QoS value in Deco app and increase it to 900 for both Download and Upload bandwidth: How to set up QoS on the Deco

This is another common issue with Deco mesh running in router mode, the value get stuck on what was the Internet link speed before the speed upgrade. 

This document also describes how to set device priority - skip that part. Do not set priority to devices at this point.

  @Alexandre. Great did that and it has improved slightly. One question though if I replaced 3 of the M5's with x20's would this improve the speeds. Also does the speed degrade if the discs are too close to each other.

  @antonyb12000 

I have Deco M5 units with my Deco mesh, I tested their speed. Under ideal conditions, Main Deco M5 should deliver up to 475Mbps. Which means, if you are getting between 300Mbps-400Mbps on device connected to Main Deco, everything is good. 

Satellite Deco connected to Main wirelessly will deliver half of that, i.e., 150-200Mbps. Satellite Deco connected to another Satellite Deco will halve speed again, to 80-100Mbps.

The WiFi speed will depend on WiFi interference from appliances and other WiFi networks, also on Satellite Deco -  from placement of Satellite Deco. Deco which is too far from Main Deco will deliver slower speeds over WiFi.

Deco M5 supports WiFi5, Deco X20 - WiFi6. On paper, Deco X20 will be faster, but it will only be faster if you are getting decent speeds on M5. If you are getting nowhere near 400Mbps from Main Deco, replacing M5 with X20 won't change much. 

It is like driving a car through the road full of potholes - replacing that car with the sports car can't allow you drive faster. German autobahn is another story.

If you are not getting these speeds I mentioned on device connected to Main Deco, it would make sense to troubleshoot that first. So, the question for you: what is the speed test results on device connected by WiFi to Main Deco.

As for distance between Deco units, "too close" is what in meters and feet? Also, please, share "Show List" screenshot from your Deco app, and it'll tell me if your Deco units are too far or too close from each other. Example:

  @Alexandre. 

Hi,

This is screen shot from app :

This is a rough plan of the house and location of the decos, only one not shown is loft which is only connected to CCTV system

  @antonyb12000 

Your Satellite Deco M5 are very well placed. They each have good WiFi backhaul link. If someone decided their location or you did it yourself: good job.

Also, your Satellite Deco are not too close to each other. If they have had full signal for backhaul link, three bars instead of two they currently have, then it may have been a time to review if you need less of them. 

In general, when deploying Satellite Deco follow these rules: make distance between Deco units not less than than 2 meters (6 feet) and quality of backhaul Signal should be either Good or Fair (yours is all Fair).

I would recommend against replacement of three M5s with X20s. You won't get much better speeds other than maybe at Main Deco, and also you may lose some of Deco M5 features if you add X20s to the mix. Such features as "connection preference," "IoT network" and probably "Test Internet Speed."