When Symantec Endpoint Protection (SEP) installed on a Windows computer, SMB network traffic throughput can decrease significantly. If SEP's Firewall component is uninstalled, the issue no longer occurs.
Performance degradation when processing occurs in user mode space.
This issued has been fixed in Symantec Endpoint Protection which introduces a "kernel mode" caching mechanism to improve performance.
In order for the fix to work, the following "user mode" features of SEP must be disabled to prevent the traffic from entering the "user mode space".
In addition, the fix will not work if on any firewall rule you have enabled packet logging.
The fix supports Windows Vista and later platforms.