@MartinMartin 

Had conversation with someone few weeks ago on that forum, he had symmetrical Internet link, but asymmetrical WiFi speeds while running speed tests. Including quite strange faster upload speeds than download speeds. I suspected he had VPN on his device, that would explain, but he said not. 

After very extensive troubleshooting and not getting anything,and me giving up on that, he just changed his favorite speed testing tool to Ookla app and to his and mine surprise started to get good, symmetrical and consistent speed test results.

Also, in addition to everything you tried, I would suggest switching Deco mesh to Access Point mode and running it in that mode if you plan to keep it: Switching a Deco to Access Point (AP) Mode

  @Alexandre. 

Thanks for the reply! 

- I don't have a VPN as well
- I have tried access point mode
- I have only used speedtest.net but I'll download the app from microsoft store (but I do notice difference in speed when using youtube)

Could it be that the Inteno has a stronger wifi signal? It looks from my computer that the signal is about the same. Or maybe there is something wrong with my deco?

  @MartinMartin 

WiFi, just like Ethernet cable, does not care which way data flows. If you had slow, but symmetrical speed results, that could have been weaker WiFi signal. This is something else.

You said you have set of X20s. Bring one of Satellite Deco to PC and hardwire it to PC with Ethernet cable. That will eliminate anything related to how PC WiFi network adapter cooperates with Deco. See what speed test results you'll be getting on wired connection from PC to Satellite Deco. 

And, yes, use Ookla speedtest app installed on PC, and verify it (OOkla) connects to nearby server, or server hosted by your ISP. 

  @Alexandre. 

Okey, I've connected my satelite deco with ethernet cable to my computer. Unfortunately with the same result. Now I'm only using the app

  @MartinMartin 

I am going to give up on this, sorry. Perhaps, someone will have suggestions to share.

To summarize, this is what you have: symmetrical Internet link and asymmetrical speed test results on PC. These speed test results are asymmetrical for both your old router and Deco mesh, even if faster speeds are in different direction for router vs. Deco.

With Internet link of 300/300, both WiFi5 and WiFi6 protocols should be able to fully utilized it. They don't, in your situation.

The most common causes for such speed test behaviour are:

1. VPN configured on PC.

You said PC does not run through VPN.

2. Speed test app issues.

You said you tried different speed test app, same results.

3. WiFi network adapter issues in PC.

By my recommendation, you hardwired PC to Satellite Deco, which switched its link to LAN adapter. Without WiFi network adapter, the issue still there.

--------------

I am out of ideas.

  @Alexandre. 

This was me.

In all fairness I also had to buy XE75 units and ditch X20.

I never tried Ookla with the X20's but I could feel the lack of bandwidth while just surfing the web while downloading, so something was off besides the seemingly buggy speedtester I started with.

  @Alexandre. 

Appriciated that you tried! I've tried to swap the satalite and the main to see if there is any difference, but it's still the same. I'm beginning to think that the deco x20 isn't that great of a product. Great range but slow wifi.