petri[.]com/microsoft-april-updates-cause-issues-for-some-users
After installing this month’s monthly rollup for Windows 7 SP1 and Windows Server 2008 R2 (KB4493472), some users are reporting that after rebooting, they are unable to log in to their systems. This affects users that have Sophos Endpoint Antivirus, Avira, ArcaBit, or Avast software installed. Microsoft announced that it is now blocking KB4493472 for devices running these products until it finds a solution.
Windows 10
... Some users have been reporting that their machines run slowly after this month’s cumulative update (KB4493509) if they have a third-party antivirus solution installed. Users on Reddit said that Windows 10 slowed down with Avira because Windows Defender was also scanning files in real-time. When you install a third-party antivirus solution in Windows, Defender should automatically be disabled. But after the April CU, it looks like Defender is still active, leading to performance issues.