Some tips https://wiki.samba.org/index.php/Performance_Tuning ]. If you can post your actual smb.conf (ie. /var/etc/smb.conf ) it will be handy >> interesting options from smb.conf file: smb encrypt, socket options, max protocol, security, min recievefile size.
Quite handy source 
https://www.performancevision.com/blog/5-things-know-before-start-troubleshooting-smb-performance/ >> (CZ) Maxovy poznamky k smb.conf
EDIT: NAS (samba) data transfer speed problems not same issue, but linked sources are very handy.