@erniej 

I am paying for 300Mbit ISP connection, but occasionally my ISP gives more. I ran test on smartphone connected to M5 which is wired to ISP router, I got 400Mbps. Even with just opening fast.com in browser on smarpthone. Screenshot below.

My Deco mesh runs in AP mode.

Distance between smarpthone and M5 is 3 meters.

Getting strong signal on smartphone: -50dBm.

Running WiFi on 5GHz only.

Suggestions:

1. Turn off 2.4GHz band in Deco app for your Deco mesh network, try speedtest again.

2. Check if M5 are not overheating. If they are more than warm to touch, that might be a problem.

3. Check CPU load on M5, through Web interface. After login, give it a few seconds to settle down. Consistent CPU load over 90% could be sign of a problem with hardware utilization, 100% CPU load is definitely not good.

@Alexandre. 

Thanks for the suggestions.. but I'm still peaking at 70-80mbps through my M5's.. averaging about 35mbps.. and I've cycled through all of my Deco units now.
I wondered in moving my smart switches to a different wifi network would solve the problem - but switching off 2.4ghz took them all offline and I still have the same poor performance.  Not sure what to try next.

@erniej 

If you are not giving up, I suggest starting with the download test with simplified network configuration. If you see substantial improvement, you could start restoring your original network setup step by step, and see where speed drops. 

If test I am going to suggest brings no improvement, nothing else likely will.

At first, to clarify: my Deco M5 is V2, on 1.5.1 firmware, in AP mode, and as you have seen from screenshot, easily provides 400Mbps to smartphone connected to it. It is consistent result: I own my Decos for over a month, have not had issues with download speed. I used to run speedtest more than once daily initially, now just occasionally.

So, suggested steps to run the test:

1. Power off all Decos other than Main Deco. Leave just Main Deco running. It is wired to ISP router, of course. Set Main Deco to AP mode if Main Deco is running in Router mode.

2. If something is connected to second Main Deco Ethernet port, disconnect it by unplugging the Ethernet cable.

3. Turn off Guest network if it is on.

4. Turn off 2.4GHz band in Main network. Only leave 5GHz.

5. Change Main Network SSID to something different. That will disconnect everything from Main Deco connected to it by WiFi.

6. Reboot Main Deco (power off, then power on). Give it few minutes after light goes green.

7. Take smartphone, turn off Mobile Data in it.

8. Connect smartphone by WiFi to new SSID you set for Main Deco. Stay close to Main Deco, but do not put smartphone on top of it. Keep about 1-3 meters distance, and keep Main Deco in plain sight from smartphone.

9. Run Deco app speedtest and confirm you are getting fast download speed, around 900Mbps.

10. Immediately after, open browser on smartphone and run fast.com in browser.

If fast.com reports speeds way in excess of 100Mbps, something must have been wrong with your original configuration. If you still have slow download speed on smartphone after you've done all of that, it is almost definitely Deco problem and you've done all legwork to prove that to Deco tech support.