@Danny_W 

>if I connect to the first M4 that wired into the same switch I barely get above 300mbps even when standing right next to it.

If I read this correctly, you are saying that wired connection shows gigabit speeds, but wireless devices connected to M4 do not get above 300Mbps.

From your home network description, I can also assume this is your Main Deco, because it is the only one that wired.

If this is true, the basic troubleshooting will be to run speedtest from inside Main Deco, to see what speeds it gets over the Ethernet cable. Then, if speeds are close to gigabit, to disable 2.4GHz on Deco Main Network and see if wireless devices forced to use 5GHz can do better.

To run speed test from inside the Main Deco, use "Test Internet Speed" option in Deco app:

Then, disable 2.4GHz on Main Network, have WiFi device connected to it and to Main Deco, check in Deco app that device is connected to Main Deco and uses 5GHz, and run speed test on wireless device.

Note that you should keep wireless device about 2 meters away from Main Deco when running speed test, and have clear view of Main Deco from where you are with wireless device.

@Alexandre. 

I'm not sure that the Deco application has the functionality for a speed test when you have an M4 connected to it, looks like that's on the M5 and later. 

When connected directly to the main M4 via ethernet I'm getting around 300mbps, and similar speeds again when connected to 5Ghz and standing 2 metres away. Yet if I connect directly to the switch that the main M4 is connected to I get in excess of 900mbps. I would expect some speed degradation, but not to this extent. 

Danny_W wrote

@Alexandre. 

 

When connected directly to the main M4 via ethernet I'm getting around 300mbps, and similar speeds again when connected to 5Ghz and standing 2 metres away. Yet if I connect directly to the switch that the main M4 is connected to I get in excess of 900mbps. I would expect some speed degradation, but not to this extent. 

You are correct, this level of speed degradation is unexpected. Also, when you run Deco in AP mode, it is just pass through for Internet traffic: no content blocking, filtering, throttling. You should be getting way better speeds on wired connection to Main Deco M4 second Ethernet port, also on wireless on 5GHz. 

>I have a Virgin Media 1Gig connection which I'm running through a Superhub 4 (in modem mode) into a ER4 router. From the ER4 I have a switch and then the first Deco M4 plugged in to this.

If you'd like further recommendations on troubleshooting, I would suggest eliminating networking gear one by one between M4 and Internet, to see if bottleneck can be found. Of course, this is just for troubleshooting purposes, you can revert back to original configuration after you are done.

I would also suggest, for troubleshooting, when you wire Main Deco to use Ethernet cable that came with your Deco set.

1. Connect Main Deco directly to ER4 router.

This is to bypass "a switch the first Deco M4 plugged in." After Main Deco M4 is connected directly to ER4, run speedtest with wired device connected to M4 second Ethernet port, as you did. That type of speedtest is proper substitute for "Test Internet Speed" option which you are right, is not in M4. My bad.

2. Configure Deco mesh to run in router mode, connect Main Deco M4 directly to Superhub 4.

If connecting M4 directly to ER4 router didn't improve M4 Ethernet throughput, the next step is to eliminate ER4. You can only do that with Deco mesh in router mode.

In case that does not improve things either, review TP-Link support article Using QoS feature of Deco and manually set value of QoS parameter "Download Bandwidth" to 900. Run speedtest again.