Open Task Scheduler.  Expand Task Scheduler Library -> Microsoft -> Windows -> Windows Defender

For each task listed, do the following:

1. Right click the task and click Properties.
2. Uncheck "Run with highest privileges" on General tab
3. Uncheck all check boxes on Contitions tab
4. Hit OK.

Your CPU usage should drop for the Antimalware Service Executable process.  If not, then reboot and it should be better.