We too are experiencing this issue. I have started a discussion on the Symantec Communities.
Slow 10Gbps network when Symantec Endpoint Client is installed | Symantec Connect
Curious if anyone else has a permanent solution that doesn't leave our operating system vulnerable. We are considering scratching Symantec and trying another solution but would like to use what we already have in place if we can. I currently have an open case with Symantec and it is in the process of being escalated.
Bottom line is with SEP installed our network throughput is slow, uninstalling Symantec clears out this issue.
Darren