When Symantec Endpoint Protection 12.1 RU2 MP1 (SEP) through 12.1 RU4 are installed on a Windows computer, SMB network traffic throughput can decrease significantly. If SEP's Firewall component is uninstalled, the issue no longer occurs.
This issued has been fixed in Symantec Endpoint Protection 12.1 RU4 MP1 and newer, 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 on any firewall rule which has packet logging enabled.
The fix supports Windows Vista and later platforms.