File transfer over SMB slow

I am trying to get the expected performance out of a 10GBit NAS, a Terramaster F8. Transfers via SMB top out at 50MB/s, with long pauses between those peaks. FTP transfer tops out at 1,1GB/s. I have tried to employ the tip mentioned here:

Slow SMB browsing - Apple Community , adding

readdir_attr:aapl_max_access = no

to the smb.conf file on the server. But that did not solve it for me. Any other suggestions would be very welcome.

On Windows, SMB transfer speeds are ok.

Yes - the connection setup is

1. Terramaster F8 connected to TRENDnet TEG-S750 10G switch via RJ45 40Gps cable.
2. 40Gbps cable from switch to a Sabrent Thunderbolt 3 RJ45 to USB Ethernet adapter 10Gps
3. USB adapter connected to MacBook with Thunderbolt 3 cable.

I am confident the hardware setup is capable of 10Gbps, as is indicated by the FPT and SMB on Windows throughput.

At the moment we are just testing this setup, it is not used in production. If I cannot get it to work faster, we will not deploy it.

I seem to have trouble with the forum functionality here. Please see my answer above.