@Galung 

Hey

This is a common enough issue that people raise, sadly however the answer may not be what you are wanting to hear!

The issue isn't the cable connection to the PC, but rather the Wireless connection to the Decos (see below in red).   The PC being a cabled in will always get as fast as possible, there is no issue here. 

The issue is further up the chain, keep in mind the Room 1 Deco needs to connect to the Main Deco wirelessly..  when doing this its providing internet for your PC and any WiFi device connected too it.  As the PC is wired in it will be capable of pulling 1000 mbps from the Room1 Deco, say you also have a Laptop connected and its pulling 500mbps from Room1.   However the connection from that deco back to the main one may only 400mbps due to the range, walls, interference etc... this is the slow bit

When you power on the PC it will become the primary resource hog as it has the fastest route to the Room1 Deco.. it will therefore cause most of the saturation on the 400mbps link.  Technically it could fill it twice over..

Furthermore the 400mbps is also being shared with the other Room2 Deco and only adds to the load

Its a bit like 2x dual carriageways merging into a single lane carriageway.   Its a bottleneck

OK so what can you do...  This depends on money and your Internet speed

If your internet speed is a good bit slower than your Deco speed (the 400 bit), then that's likely not an issue and its something to live with.  If the deco can get the devices to your internet quicker than it can handle, then IMHO its not be worried about.   To test this run a speed test  (FAST DOT COM) and see if it matches you internet speed, if it does all good.   At a guess if your internet is over 300mbps then the Deco may be a bottleneck, less you are likely ok.  However that is just a finger in air guess

Option 2..  Look at returning / upgrading these to the Deco X20 X30 etc range..   The X range is the newer standard of WiFi and offers much faster speeds, therefore that 400 could be 600 on the new hardware.    However the hardware is new and pricey!   Its never going to be perfect as WiFi is always slower, but it should be much faster.   Again only justified if your internet speed is 300mbps or more.

Option 3..  Run a cable from the ISP Router to the room1 / room2 deco and switch them to AP mode.  Yes this is messy!  but if you have internet over 500mbps this is your only option as WiFi simply wont do those speeds.   It removes the slow "wireless backhaul" issue all together.

Hope that helps!!!

  @Philbert 

hi Philbert, thanks for the lengthy explanation

Currently im on a 100 Mbps package plan on my ISP, so the said speed is lower than that of the 400 Mbps you mentioned. In my case, the speed that is offered wirelessly through all the deco units don't seem to bottleneck unless i turn on my PC (in which you stated if i do turn on my PC, it will the primary hog). it's true that if it does bottleneck, it's not really much of a problem. The problem is the ethernet internet speeds from said deco 1 unit to my PC are not consistent, by that I mean there are lots of spiking when im gaming, communication through discord and even watching youtube from time to time. 

I did consider trying option 3 for quite a while, though it will be a shame since I initially bought Deco as to prevent routing cables in and out through different rooms as it is quite a hassle XD. Maybe this time I'll consider it. Currently I have 2 questions in mind

1. in the case of room 1 deco, should i connect the cables to my ISP router itself, or connect to the main deco (which i already connect to my ISP router)?

2. by switching to AP mode, what difference does it make compared to being in router mode?

Other than that, your explanation really helps. Thanks

  @Galung 

Hey

Yeah if you are gaming the WiFi link between the main deco and Room 1 will be what is causing you spikes.  While its likely ok for general stuff, WiFi isnt great for gaming as it can be subject to ping spikes and massively increased latency, sadly just the result of using Airspace to transmit data!

Bypassing the airspace by using a cable should sort this, gaming is best cabled in (as a gamer myself :) )

Im answer to your questions, i do it in reverse

2. by switching to AP mode, what difference does it make compared to being in router mode?

OK.. so in router mode the Main Deco becomes the central point of your network, everything goes through it and it hands out IP addresses to your devices.   This allows you to use it as the main node and therefore connect the other Deco wirelessly as you have done.  The only time your ISP router is involved is if the Deco needs soemthing from the internet.   The  ISP router will fetch it, tell the Main deco and he does his thing.  This is likely the way Deco is setup for 99% of people, advantage is its easy to setup and supports wireless backhaul.

Access Point Mode changes that..  Rather than being a MAIN deco with attached nodes, all the Decos work as equals and your ISP router is the main hub.    As they are equals they wont talk wirelessly, instead will need cabled into eachother, or the ISP router via one of hte Deco LAN ports.     The advantage is that you have a wired backhaul, so the full WiFi antenna is now available for clients.   The other free port on the Deco can be used for your PC also, so PC out to Port 2 on Deco, out Port 1 on deco to Router and Internet.. no wifi.. 

It literally turns the deco to APs

hope that helps?

1. in the case of room 1 deco, should i connect the cables to my ISP router itself, or connect to the main deco (which i already connect to my ISP router)?

If you are going to cable you need to use Access Point Mode and ideally cabled back to your ISP router.. however you can daisy chain them if needed but it uses both ports for that..   There is no Main Deco anymore in Access Point Mode..   this link might help also

https://www.tp-link.com/us/support/faq/1842/

  @Philbert 

thankyou for the explanation, I'll see what I can do. much appreciated! :D