@David-TP Hello David and thanks for your reply.

Your diagram is correct except the Mac is connected on the main Deco, and yes, the measuring units are in Mega bits, please see the attached screenshot.

Connecting to the same satellite physically is a bit tricky since I'm lacking the proper TB to Ethernet adapter (I am using a wired dock on my office desk, it can't be moved).

So I tried to conduct other tests that might help:

1. Connected a wired backhaul between the satellites (a long LAN cable), performed the same test, pretty much no change (Internet speeds are now top speed on both units).

2. Moved the MacBook Air near the remote X90 Satellite and verified it is connected to it via WiFi (using the Deco app), now the write speed has increased dramatically to 500Mbs.

3. Took the MacBook Air back to the main X90 unit, and verified it has indeed connected to it, now the speeds dropped again as before.   

Still I am trying to understand from you, what are the nominal speeds I should expect with an optimum range between the units (right now in the app, it shows Strong signal to the main unit).

Another piece of information is about the X90 installation process:

Before the X90 I had Deco M5 units installed. So I have joined one X90 in and then switched it to be the main unit in the network via the app,

then added the 2nd X90 unit successfully. 

After both new units joined my network okay, I removed all the M5 Deco units via tha app, leaving only the X90 new units standing.

I did that to make sure all my clients will not lose their ability to connect to the new units and keep all my settings in place (I knew that keeping the the current SSID and password might be enough, but wanted to be sure).

I couldn't find any documentation on how to migrate properly between one Deco model to another, so I hope I did it correctly.

Again, thanks for looking into this.

-Ran