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Following fixes are mentioned in SEP 12.1 RU3 notes:
Network performance slows on Windows XP with Symantec Endpoint Protection 12.1.2 MP1 installed
Fix ID: 3180923
Symptom: Computers experience network performance problems when the Network Threat Protection (NTP) firewall component of the Symantec Endpoint Protection 12.1.2 MP1 client is installed and enabled. Symptoms include:
•Copying files from a network location over Windows File Sharing takes significantly more time.
•Programs that are started from a network share take significantly longer to load.
•Ping responses take longer to return.
Solution: Modified the firewall to call the correct notification routine for Windows XP.
Symantec Endpoint Protection Manager performance is slower after upgrade to 12.1.1 MP1 or later
Fix ID: 2882607
Symptom: After upgrade to Symantec Endpoint Protection Manager 12.1 RU1 MP1, performance of the Symantec Endpoint Protection Manager server is degraded. Delaying the security mining task (scm.timer.securitymining.delay) resolves the issue.
Reference: New fixes and features in Symantec Endpoint Protection 12.1.3
For 800 clients installing SEPM on Windows XP machine can have some OS limitations.
8.2 GB database size for 800 clients is acceptable.
I would suggest to install it on Server operating system if possible.
Maintaining the database
http://www.symantec.com/docs/HOWTO55337
Disk Space Management procedures for the Symantec Endpoint Protection Manager
http://www.symantec.com/docs/TECH96214