High CPU usage by Antimalware Service Executable (Windows Defender) on every login

Hi HeavenlyMistakes​,

I’m assuming you’re on Windows 10, based on the tag. This is a known issue where Microsoft Defender can run a scheduled scan or get stuck rebuilding its scan cache right after login, which can cause MsMpEng.exe to use very high CPU even if exclusions and limits are already set. Microsoft has documented this behavior, especially after Defender updates.

A commonly recommended fix is to reset Defender’s scan cache. Stop the Microsoft Defender Antivirus Service, delete the contents of C:\ProgramData\Microsoft\Windows Defender\Scans, then start the service again and reboot. If that helps, you’ve likely found the cause. You can also temporarily disable the Windows Defender Scheduled Scan task to confirm it’s a scan running at logon, while keeping real-time protection enabled.