In macOS 10.11 and 10.12, SMB signing is turned on by default. This is no problem for gigabit ethernet, but SMB Signing severely limits the performance of 10 gigabit ethernet. Apple has published a tech note how to turn off SMB signing: "Turn off packet signing for SMB 2 and SMB 3 connections". This note requires the use of the terminal application and a number of terminal commands, so some familiarity with those tools is required.
In macOS 10.13.6+, Apple has turned off SMB signing by default.