To fix high memory usage by Antimalware Service Executable on Windows 11, try turning off real-time protection, uninstalling pirated apps, scanning for malware, rescheduling Windows Defender scans, or ...
I just noticed on my mom's computer that some "MsMpEng.exe" process (labeled "Antimalware Service Executable" and located under "C Program Files\Microsoft Security Client\") is consuming 100% of CPU.