[Samba] samba 4.x slow ...

[Michael Arndt]

just to verify basic facts:

Did you cross check vie network sniff, on which SMB protocol versions Server + Win 7 clients agree ?
Or did you pin down via registry ?

AFAIK only starting with win 8 or win 10 clients you could ask with powershell, which protocol version is in use.

Did you also cross check  samba logs for a name resolution issue ( windows names, not DNS)
if one of your boxes is an commercial appliance alignment issues may be excluded ...
if you get reasonable throughput  via NFS, this points to  "where is a difference in your environment between NFS and SMB / CIFS).

hth
Micha