system getting slow after download the virus definition, how to resolve it?

How many system getting slow?

Turn off the Active Scan

How to turn off Active Scan when new definitions arrive

High I/O disk using Endpoint Protection on Windows 8 machine

It's managed SEP client or Unmanaged SEP client ?

If managed Client See below articles

How to turn off Active Scan when new definitions arrive

About the file cache

If Unmanaged Client See below steps.

Try this and change the registry entry

You must set the following registry value: HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\AV\Quarantine\DefWatchMode=3 (REG_DWORD).

DefWatchMode
value action
0 Automatically repair and restore files in Quarantine silently
1 Repair the files in Quarantine silently without restoring
2 Prompt user
3 Do nothing

If you wont be able to modify the registry if the tamper protection is enabled. disable it for a min make the changes

Enabling, disabling, and configuring Tamper Protection in Endpoint Protection on unmanaged Clients

system getting slow after download the virus definition, how to resolve it?

Turn off the active scanning while applying the new defintion.

The following steps will change the policy configuration to disable these Active Scans.

1. Open the Symantec Endpoint Protection Manager (SEPM) console
2. Click on the Policies tab on the far left side
3. Highlight Antivirus and Antispyware under the View Policies column
4. Highlight the policy in question under the Antivirus and Antispyware Policies window. If you have not created a new policy, select the existing policy.
5. Within the middle column under Tasks, select Edit the Policy
6. A new window will open.
7. From the new window, there will be a menu on the left side, select Administrator-defined Scans
8. From the Administrator-defined Scans window on the right side select the Advanced tab
9. Unselect Run an Active Scan when new definitions arrive
10. Click OK
11. Either update the policy from the manager/client, or wait for the next update cycle

Is an active scan running when new defs arrive? Check your scan log to confirm

Yes active scan was enabled. Turn off and problem solve.