@David-TP 

Yes, I checked the wireless speeds on my phone connected to the base Deco at 5 GHz from 2 meters away in the same room and was seeing similar results - perhaps a slightly higher average of around 290 Mb/s or so.

Are you saying that the best that I can reasonably expect given my setup is roughly 350 Mb/s?

I'm not expecting to get full speed via wireless, I know there are overheads and environmental influences but a drop off of 67% feels off.

  @CampFreddy 

If you want to find what your Deco M4 is capable of, in your environment, you will need to run speed tests with the following setup:

1. Switch ISP router to router/gateway mode;

2. Switch Deco mesh to Access Point mode: Switching a Deco to Access Point (AP) Mode

3. Power off Satellite Deco(s), leaving just Main Deco running;

4. Switch Deco Main Network to run on 5GHz only:

5. Verify that link speed on smartphone connected to Main Deco is 866Mbps down and 866Mbps up. If less, move smartphone closer to Main Deco. https://www.tp-link.com/en/support/faq/2265/

6. Try different speed test utilities, Ookla among them is recommended.

What you'll get in that setup should be the maximum of what Main Deco M4 is capable. 

You can undo all these changes after you ran your speed tests.

  @Alexandre. 

I did as you advised and am still not getting more than 300Mb/s (multiple tests, so that's a rough average, but the numbers were around that and not massively higher or lower), even with the phone physically sitting on top of the base M4.

That was a quick set of tests as I cannot spend a lot of time on it tonight, but I will try again tomorrow evening.

  @Alexandre. 

Apologies for the delay. I did what you suggested.

Router (Virgin Media SH5): router mode.

Deco Base Unit: Access point.

No other Decos powered on.

5 Ghz band only.

I'm still seeing no real differences in speeds, regardless of setup. With my phone physically sitting on the base Deco unit, it's still only roughly 300 Mbps on average. With the peak being around 360 Mbps.

  @CampFreddy 

This configuration I gave you removes everything in Deco mesh which could impact WiFi speeds. It disables content filtering and throttling, priority devices, antivirus and so on. It also forces device run on 5GHz all the time, so that Deco mesh couldn't force it to run on slower 2.4GHz to save device battery power.

I know this is not what you want to hear, but if you set your Deco mesh the way I said you should for speed testing, speeds you are getting is the best you could have at your place. I can't explain why they are that low, you should see close to 500Mbps-600Mbps from M4. What I can tell you quite confidently is whatever else you try to do with Deco mesh settings alone, won't improve its WiFi speeds.

For example, there might be strong WiFi interference around Main Deco, and that Deco mesh could not fix.