I went ahead and tried what you did, and surprisingly, the overhead is present at the slower connection over WIFI too.
Wired file browser transfer = ~44 MB/s
Wired cifs mount transfer = ~80 MB/s
Wireless file browser transfer = ~4,6-5 MB/s
Wireless cifs mount transfer = ~5,6-6 MB/s
Tested with different files on same disk so as to avoid cached material.
Edit: Samba version 4.3.11-Ubuntu
And, the processor load on the server is very much the same for the two kinds of transfers.
Edit: On Windows 10 it reports a speed of 105MB/s, but I am unsure if it is Mibi or Mega. Anyway - it is fast.
I have been reading a bit on Netbios, SMB3/SMB3, Cifs and such. But to be honest it is a bit over my head. For one, either articles say CIFS is old, slow and insecure, or they say Cifs is new.