Windows Defender consuming 40% CPU even though folders are excluded

Hello @Foo Yuen

Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

Based on investigation we can suggest detailed lists below:

Please confirm that the Defender Scheduled Scan task is disabled if you don't wish to scan it.

Common reasons for higher CPU utilization by Microsoft Defender Antivirus : https://learn.microsoft.com/en-us/defender-endpoint/troubleshoot-performance-issues#common-reasons-for-higher-cpu-utilization-by-microsoft-defender-antivirus

Now check in PowerShell for exclusions (Set-MpPreference) and let's confirm all the necessary folders, extensions, or processes are excluded also refer the detailed suggestions from Microsoft : https://learn.microsoft.com/en-us/defender-endpoint/configure-exclusions-microsoft-defender-antivirus

High CPU usage due to Windows Defender (MsMpEng.exe) : https://learn.microsoft.com/en-us/defender-endpoint/troubleshooting-mode-scenarios#scenario-2-high-cpu-usage-due-to-windows-defender-msmpengexe

Please Run Process Monitor (ProcMon) during these spikes to see which files or folders the service is accessing, then add targeted exclusions.

For troubleshooting performance issues related to real-time protection : https://learn.microsoft.com/en-us/defender-endpoint/troubleshoot-performance-issues

Kindly let us know if the suggested steps helps or you need further assistance on this issue.

Regards

Himanshu