Sometimes an OS X Server will have very poor SMB file sharing performance – whilst I haven’t been able to ascertain with 100% certainty what causes it, something that may be a factor is the use of SMB 3 connections over SMB 2.

SMB 3 connections can be signed and encrypted, and this can put a significant amount of overhead on the server.

To disable SMB 3 at the server end, type the following in to Terminal

sudo scutil --prefs com.apple.smb.server.plist
get /
d.add ProtocolVersionMap # 2
set /
commit
apply
quit

Info from https://support.apple.com/en-au/HT204021