@Solja 

Deco M5 has 1Gbps Ethernet ports and can do much better than 100Mbps with Ethernet backhaul. I have Satellite Deco M5 wired directly to ISP Gateway, I am getting 300Mbps download speed on smartphone connected to it on 5GHz. My Internet connection is 300Mbps download/20Mbps upload.

If you can tell how do you run your Deco mesh, in Router mode or Access Point mode, and if you wired your Satellite Deco to modem or to Main Deco, I may offer troubleshooting steps for you.

Also, please run "Test Internet Speed" from inside Deco app on smartphone, it is under More section. This will run speedtest between yor Main Deco and Internet. Are you getting numbers close to 1Gbps?

@Alexandre. 

I'm using the deco in router mode. In the tp link app speed test I'm getting close to 800mbps. 

What's strange is that when using the following setup, I am able to get just over 200mbps.

Main deco --> wireless backhaul --> deco 2 --> cat 6 cable --> laptop. 

I tried this with both ports on the deco 2. 

So both ports on the deco 2 are able to go over 100mbps. 

Both ports on the main deco are able to go over 100mbps. 

As soon as they are connected in wired backhaul, they seem to remain at 100mbps.

@Solja 

I am almost inclined to say you have 100Mbit switch somewhere between Main Deco and Satellite Deco.

Now that you confirmed your Main Deco is getting 800Mbit from the Internet, you could test your wiring. You proved that Main Deco can get good download speed.

1. Power off all Decos.

2. Bring Main Deco to where your wired Satellite Deco is, disconnect Satellite Deco from backhaul Ethernet cable and connect Main Deco to it.

3. Make sure that other side of that backhaul is now connected to modem.

4. Turn on Main Deco and wait till it gets green light.

5. Run speedtest from Deco app.

If you'll be getting 100Mbit in that setup, it is definitely wiring.

If you are still getting 800Mbit, see this TP-Link FAQ article: Using QoS feature of Deco

@Alexandre. 
 

Okay I tried your suggestion and something really weird is going on.

See image below. 
In this setup, cable 1 is connecting the main deco to the modem. 
Cable 2 is connecting main deco to deco-2.
Cable 3 is connecting deco-2 to laptop. 
 

I am getting full gigabit speed. This (to me) implies that all 3 cables are operating effectively.
 

HOWEVER: When using cable 1 between main deco and deco-2, I then get limited to 100mbps on deco-2. Main deco still functions at 1gbps (implying the cable between modem and main deco still functioning correctly. 

So I'm a bit baffled why the same cable can support gigabit speeds between modem and main deco, but not between main deco and deco-2. 
 

Note: I tried swapping around the ports and this made no difference.