Hi,
You can improve the performance with the help of SEP 12.1 features.
SEP 12.1 has many new features.
It supports advance virtualization support with the help of following features.
1) Virtual Image Exception – Allows to exclude all the files on a baseline image from scanning.
2) Shared Insight Cache – A stand alone server that enables clients to share scan results. This allows clients to skip scanning files that have already been scanned by another client.
3) Virtual Client Tagging – Makes the clients virtualization aware and sends back the hypervisor vendor to SEPM. That data can be used in client searching and reporting.
4) Offline Image Scanner – A stand alone tool to scan offline VMware image (VMDK )files.
Check this article: Symantec Endpoint Protection 12.1 & Virtualization
http://www.symantec.com/docs/TECH194383