Ntrtscan.exe process causes high disk IO in OfficeScan

This issue is encountered on computers installed with applications that have large files like SQL and VMWare.
The workaround solution is to disable the digital signature cache on the affected machine. To do this: 

1. Open the OfficeScan server's web console.
2. Click Networked Computers > Client Management. 
3. Click Privileges and Other Settings for the affected machine/group.
4. Go to the Other Settings tab.
5. Under the Cache Settings for Scan, remove the checkmark on Enable the digital signature cache.
6. Click Save and wait until the changes are applied.

Please see article: http://esupport.trendmicro.com/solution/en-us/1060249.aspx for my detailed information.