@TomerA 

Are you running the Deco in Acess Point mode?

If not it will likely ignore the cable backhaul and try to talk wireless, hence the slow speeds.

For your setup use Access point Mode.

@TomerA 

Hi,

I'm not sure how to check/change that setting.

This one of the other Deco if that helps.

Anyway I encounter the problem also when I'm connected to the main Deco.

@TomerA 

Hey

Thanks for the screenshot, that helps! 

You are already connected via ethernet, so it is using the cable connections between the nodes.  That rules out Access Point Mode as the issue

Can i ask how you are testing the speed?  is this what you are referring too?

@Philbert 

no, i'm checking in Ookla Speedtest. I ran 10 different tests on different devices. Even changed the test  server so they will be the same. The results are consistent. 

@TomerA 

I also tried disconnecting the switch. So I'm just testing the Deco wirelessly. Got the same res

@TP-Link 

I have it for 10 months now.

My total internet bandwidth is around 50M/s--download/5M/s--upload

I have 2 desktops, 2 mobile phones, 4 tvs, 2 cameras, 4 sensibo devices (for ac), 2 tablets.

My ISP does not provide a fiber cables on my area so this is as fast as they can perform.

I did had a very good connectivity until 2-3 months ago.

@TomerA 

Hi, any ideas?