@NetanelC 

On your PC, check what Ethernet speed it negotiated with Deco X20 it is wired to. See How to check the associated/link speed on a wired client

If PC negotiated 100Mbps, you'll need to focus your investigation on PC Ethernet network driver and on Ethernet cable between PC and X20. Deco does not have configuration settings to downgrade Ethernet link speed from gigabit to 100Mbps.

  @Alexandre. 

The setup is  -  ISP >cable> Deco >cable to female socket> cable to whatever device I choose. 

When I connect my laptop to that cable I get 1GB connection.

When I connect my PC to the same cable I get 100Mbps.

When I disconect the Deco (now setup is - ISP >cable to female socket>) and connnect the PC straight to the ISP I get 1GB conection. 

I still get around 200Mbps on Wi-Fi even though the deco is now connected wirelessly but that's not ideal as before I would get around 600Mbpps. Seems like it's an issue with the Deco. Any suggestion? 

  @NetanelC 

I may be able to suggest workaround, which is to rearrange your network layout, but I need more info from you before I can do that.

You must have more than one Deco X20. If you do have more than one, where are the rest of them? Please add them to your network setup description or drawing.

When you say "connect the PC straight to the ISP" - do you have ISP modem or ISP router/gateway? If you are unsure, share its brand and model name/number, and I'll check for you.

I asked you to check on PC what speed it negotiated with Deco, have you done that? If yes, what is link speed reported by PC network adapter?

  @Alexandre. 

Full setup and the way everything is wired  -  ">" is cables, "female to female" is wall-wired cables

ISP (GR140DG-Fiber gateway)

   > Deco1

   > female-1stFloor to female-2ndFloor > Deco2 > female-2ndFloor to female-office > whatever device... 

      Deco 3 is connected wirelessly.

Currently, because it seems to cause an issue with the PC, I disconnected Deco2 and plugged a short cable to bridge the two wall-wired cables  -  the one that came from the gateway and the second that going to my room. Including all the in&out cables, my PC now has five cables that are connected straight to the ISP. i'm getting almost 1,000 Mbps. 

It's gonna be hard rearranging the layout. It's a brick house so every wiring requires making holes and channels. It's a complete construction mode.

Also, I can't change the location of my ISP as we had issues getting the fiber past where it is now. Ideal situation it will be on the 2nd floor and every device connects straight to it, with the deco  being used purely for Wi-Fi rather than being used as a switch .

Update - just noticed that whenever I turn off and on the computer, it shows 100Mbps. When I disconnect the cable and reconnect it goes back to 1GB. that "fix" doesn't work when the Deco is connected.  

By the way, it is a new PC. I did update all the drivers and Windows updates are up to date but maybe I missed some kind of setting? 

  @NetanelC 

Looking at your network diagram, I have a question: do you run Deco mesh in Access Point mode? If you have Deco 1 and Deco 2 connected to the gateway by Ethernet cables, that is only going to work with Deco mesh in AP mode. Please confirm.

Also, in that mode there is no QoS option. You are supposed to get maximum Deco performance, all bandwidth throttling and content filtering settings are disabled when Deco mesh runs in AP mode.

As for workaround, my suggestion is to get inexpensive unmanaged 5-port gigabit switch and put it to split connection between Deco 2 and PC. 

Instead of what you have now:

 > female-1stFloor to female-2ndFloor > Deco2 > female-2ndFloor to female-office > whatever device... 

Make it to this:

 > female-1stFloor to female-2ndFloor > switch > female-2ndFloor to female-office > whatever device... 

                                                                    I-------> Deco2

I would recommend to get switch that is compatible with Deco Ethernet backhaul. The topic of switch compatibility is discussed here: General questions about Ethernet Backhaul feature on your Deco

  @Alexandre. 

It is Access Point mode.

It might be what I'll eventually do. Haven't found a nice 2-3 ports switch and I'm not sure yet if I want a bulky one and more ports than I'll use. 

Any way.. I guess I'll just need to forget about it. It's not ideal but I still have great speeds on PC and Wi-Fi.

Thanks for the advice and I appreciate the patients. To be honest, I didn't think anyone was gonna reply to the post :)