@JonathanKnowles 

Hi, Welcome to the community.

Is 200m/s the Ethernet speed directly from the main Deco M4? Or it is actually the wireless speed from the main M4?

If it is the wireless speed, could you please help me check the wireless link speed on the tested device:

How to check the associated/link speed on a wireless client

Please make sure the QOS has been disabled or set to 1000m/s download/upload.

Thank you very much and best regards.

  @JonathanKnowles 

Hi, I believe BT router could also provide Wi-Fi connection?

So "the speed to the device itself is around 200 (again supplied by the BT app. If I run speedtest I get similar values. Just ran it now and actually got 100!"

Could you please help me confirm whether these two speedtest results from all from BT WiFi router, or 200 from BT, 100 from Deco?

Since Deco is connected behind BT router, its internet bandwidth will not be higher than the BT router bandwidth.

As for the TX rate, It is the maximum possible transfer speed you can expect from the router at that time. It is the transfer speed that the router can send, which is quite considerable since the whole bandwidth is about 500m/s. 

Link speed is what your computer is actually able to receive and will be affected by the distance from the router and possible interference in the household. So appart from the TX rate, how about the link speed on the tested client?

Best regards.