@Mdevente 

I guess it could be either a problem with poor quality mains wiring in the house so the powerline signal is getting badly degraded, or the satellite P9 has decided to connect to the main unit via wi-fi and not powerline, but with a poor signal.

How far is the bedroom from the main P9 unit?

You could try placing it closer to the main P9 unit and see if the speedtest improves.

For me this another example of why the firmware and app need improving so we can analyse which data transport streams the P9s are using.

And then the ability to override the automatic routing selection, to force the units into communicating between each other using powerline or wi-fi.

It has been asked for in many previous posts, with a reply that it is being considered for the next frmware release.

But it seems nobody at TP-Link is able to say when the new firmware will be available and what enhanced (or not) feature set it will contain 

@lohr500 in afraid it's also due to my power wiring. I tested an older AV500 powerlink and that one has even less speed left. Bedroom is on other side of house and 1 level higher. Also main router is on left side and slave on right side of house.  (So front left VS back right)

It's a house of the 70's and previous owners have installed a lot of cables towards garage and garden. so I guess it's logically that power grid isn't great.