Further to our initial investigations we are applying a pair of updates to test machine to address an issue in which system performance can be decreased during scans. This issue has the greatest effect on computers that have a small amount of physical memory. We’ll be monitoring the machines for the next few days to see if they continue to exhibit the same symptoms. We’re concentrating on reviewng a couple of fixes that Microsoft have produced to resolve issues around machine performance relating to the update services.
Further infomation on them is at https://support.microsoft.com/en-gb/kb/3050265 and https://support.microsoft.com/en-us/kb/3102810.