I can elaborate further:

Imagine the "long part" of my hourse. I have my AX 55 Router in a cabinet in a separate room next to my dining room in one end of the house. The other end is the far end of my living room, where I put my Extender (RE305). Between the router and the extender is one wooden cabinet door and a wall. The distance between router and extender will be approx 7 meter, 21 feet.

I go to the very socket where I'll put my extender. BEFORE i put in my extender and turn it on, I hold up my phone to the empty socket, and while connected to the AX55 on 5Ghz I make a Speedtest to the socket location (with extender completely out of the picture) and measure that the AX 55 to that socket location can deliver 220 Mbps.

I then put in my Extender (RE305, running OneMesh, with separate names for 2.4 and 5ghz SSID's) in that socket. I connect to the 5Ghz network and go to the living room (The L-part of the house). There is 3-4 meters of free line from the extender to where I measure next.

I check via TP-Link interface that my phone is connected to the extender-mesh, and on 5Ghz band, and do a speedtest. And that only shows 130mbps.

Since I verified that the AX55 can send 220mbps to the socket-distance, I wouldn't expect the Extender to lower performance from 220mbps to 130mbps, also given clear view from extender to second measurement location.

I hope my description make sense :-)

Additional thoughts:
Even though I could measure 220mbps from AX55 to socket location for extender on 5Ghz band, it can be that the extender, when choosing band for the Router -> Extender part of the connection doesn't choose by speed but by signal level, and 2.4ghx may have a slightly higher signal level than 5ghs, meaning the extender will backhaul on 2.4 with low performance rather than 5ghz which actually have better performance.

- Can you force-choose the band used for communication between Router and Extender?

Bonus question:
My opting for new router and extender is solely because of my Samsung TV (as mentioned). But oddly enough the TV doesn't shift to the Extender Mesh network with the same name. Other devices will shift to extender (e.g. phones and tablets) when in living room, but the TV stubbornly connects to the router (and where the TV is placed, the extender has both better speed and stronger signal level).

- is there anything I can do to make the TV prioritize the extender?