@Nick_CY 

Yes, this is ugly. Deco mesh can do better, especially in AP mode.

To help me understand what you have, make screenshots of "Show Map" and "Show List" pages in Deco app and share them here. Example:

@Alexandre. here it is. 
The problem persists when connected to either of the two devices

  @Nick_CY 

Screenshots say everything is OK and link between two Deco units is good.

Do you have device with Ethernet port you can hardwire to Main Deco and run pings from that device? 

  @Alexandre. yep

​​​​much better via ethernet but worse that without x50

  @Nick_CY 

Here is the thing: if it were just WiFi interference, you should have had excellent wired ping. If it were problematic Ethernet cable, you should have continued getting same bad pings from hardwired device.

There could be multiple issues which is almost impossible to troubleshoot remotely, or something else going on - such as (unlikely, but) defective Deco hardware. 

The only thing I would recommend is upgrade X50s to latest available firmware from here: Deco X50/X55_V1_1.5.1

If that does not fix the issue, just return Deco set to retailer.

  @Alexandre. I'm already using the latest firmware version for the X50 (1.2) and problem still persists

  @Nick_CY 

The latest Deco X50 firmware for hardware V1 is 1.5, from the link I provided. If you don't see it in Update Deco page, you will have to do manual update.

For update, use firmware from link I provided and follow Method 2 from this TP-Link FAQ: How to Update the Firmware of Deco

  @Alexandre. I got v1.20 not v1 and for my hardware version the latest firmware version is Deco X50(EU)_V1.20_1.2.0 Build 20231229 which one I already got. 
miscom in versions cause of there is no v1.2 in dropdown list when u created a thread. 

  @Nick_CY 

Almost like a bad luck with Deco mesh - they have newer firmware for V1.

Sorry, can't help you with that. Perhaps, someone from TP-Link Support will notice that topic, so just wait till tomorrow. If not, return to retailer is better option than to waste time on networking gear which does not work for you.

Deco mesh in AP mode is very simple. There is minimum configuration settings, and there are none which could impact networking performance of Deco mesh the way you experience. It either works, or does not.

  @Alexandre. Thx for your help and have a great day. Hope I can return it and buy smth else not tp-link devices. This was awful mistake to buy x50. 

It's unlikely that support will be able to help me with anything; the most they usually suggest is to disable mesh technology in a mesh system, lol.