Ethernet speed with WiFi Client

I have a problem with the speed of the ethernet if there is a wifi client at the other end of the chain.

Connection:
N2+ ---- switch 1Gb — Wifi Router (Asus 5400)-----Wifi5 Client (laptop)

The upload speeds are ok. (WiFi5 => N2+) More than 400Mbit/s
Download speeds on the other hand are terrible. (N2+ => Wifi5) Under 35Mbit/s. Measured with iperf3.

If the client is wired, speeds in both directions are over 900Mbit/s
There are no QoS or other traffic limiting rules on the wifi router.

If I connect a computer with 1gb ethernet instead of N2+, the speeds in both directions are fine (>400MBit/s)
Connection:
Laptop 1Gb---- switch 1Gb — Wifi Router (Asus 5400)-----Wifi5 Client (laptop)

Any idea why the download speed should be so low?
So far I’ve tried resetting the Asus to factory settings, replacing it with another identical piece.
But all indications are that the problem lies on the N2+ side.

Wifi is not WiFi, try another adapter, another channel,…

BUT wire is a wire

As I wrote…

If I plug my laptop into the same place instead of N2+, the speed in both directions is stably > 400Mbps

The same behavior is also in the 2.4GHz band. (The speeds are of course lower, but the ratio remains)
In the 5GHz band I am alone.

I changed wifi router and end wifi clients…