Microsoft s April 9 security update is bogging down systems running antivirus software packages made by McAfee, Avast, ArcaBit, Avira and Sophos. Microsoft describes symptoms tied to the April security update and the Kerberos implementation in several versions of Windows. McAfee and Avast suggest the problem are tied to a change Microsoft made to the Windows Client-Server Runtime Subsystem (csrss.exe) The CSRSS is a vital part of Windows, responsible for the user mode side of Win32 windows.
Source: https://threatpost.com/microsofts-latest-patch-hoses-some-antivirus-software/143978/