@Axwell_ 

The issue is related to the Ethernet cable being used so the other people as well are either using a Cat 5 and not a Cat5e or the current cable being used is bad. If a wireless client can get 200 Mbps, then you know the modem, cable from the modem, and the port of the Deco that the internet goes into is good. Another cause would be the device you are wiring to the Deco may only be capable of up to 100 Mbps.

@Tony its a cat8 ethernet cable and the device is a laptop so neither of those are the issue...its a problem with the deco and their firmware..as per other people with the same issue here and reddit. 

@Axwell_  I have the exact same issue. Gigabit all the way through but when i plug a device into the the 2nd port on the main deco (i have 4) it is limitied at 100MB. I'm running in router mode. I've confirmed its not a cable issue. I'm pretty sure TP-Link says these units have two gigabit ports 

@Tony 

With the following configuration I get 100Mb/sec

Server- gigabit switch - Deco - PC

If I plug the PC (same cable) into the gigabit switch I get 1000Mb/sec

All I can conclude is the port is 100Mb not Gigabit.

The Deco does have gigabit ports, the Deco W2400 and the E4 are the ones that have 100 Mbps ports.

The Deco should have a speed test which tests the speed from the main Deco, what speeds does that show?

Also, check the QoS speed setting, if that is set to 100 Mbps, then everything connected will be 100 Mbps. Set it to 1000 Mbps up and down:

Not sure if this post advances the discussion but since 120 is a figure above 100 here it is...

I Pay for 120 and my PC measures 120 via fast.com on any of the folowing "*PC" locations.
during the test I see numbers in 200's then things throttle down to 120 sometimes 130.

ISP/ISP_Modem
|
|cable_from_M5_box
|
M5    <<<  >>> *PC
|
|cat6
|
Gswitch--cat5E--*PC
|
|cat6
|
M5 <<<     >>> M5 <<<  >>> *PC 
|
|cat5E
*PC