Perhaps I have done something wrong, but I am getting much slower LAN speeds than I was expectig.

I've go three M4 units set up behind an EE 4G router. The Deco units are running in router mode.

To test the LAN speed, I disconnected the main unit from both the router and the wired LAN (which was acting as an ethernet backhaul to one of the other Decos plus some hardwired kit in abother room).

I connected a laptop and phone to the 5GHz network of the main unit. The laptop and phone both reported that they were connected at 866Mbps.

I then ran an iperf3 test (well, multiple tests) between the phone and the laptop (the laptp was running as a server). Most of the trasnfers were around 115Mbps with the fastest being 126Mbps.

With everything plugged in, the same text maxes out at around 95Mbps. A similar test with the full network up between the laptop (in client mode) and a Raspberry Pi (model 4+, but using a USB gigabit ethernet adapted) hits between 94 and 110Mbps.

The network itself is:

main Deco -> Deco 2 using cat 5e ethernet cables an Cat6 sockets

mian Deco -> Deco 3 using wifi backhaul (5GHz)

main Deco -> living room (containing Raspberry PI) using cat 5e ethernet, Cat6 sockets and gigabit TP-Link unmanaged switches.

The one thing I haven't yet tried is to turn off Decos 2 and 3 and just test the main router Deco completely on its own. I'll give that a go when the house is empty and I won't have family yelling at me for taking their (rather megre) internet away.

What might be the problem here? Could there be something I have done in the settings that is throttling the LAN? Or have I misunderstood something?

This is all completely academic for my Internet as I only get between 4-15Mbps down and 6-10Mbps up, so the LAN speed is still faster than that. But it does afect moving data around the LAN and running backups.