In the link below you will read all about the problem you are experiencing and advice on how to fix it. I've copied what
I think is possibly the best solution but if you upgraded from Windows 7 then you will need to read further.
Can one prevent Antimalware Executable to start hogging CPU for - Microsoft Community
You can disable the scheduled scan task by right-clicking and selecting “Disable”, but that won’t stop the other Automatic Maintenance tasks from running shortly after startup; so what you should do is reschedule Automatic Maintenance for a time when the PC will be on but not in use (by clicking on the “Change maintenance settings” link in the Action Center) and see if that makes any difference.
Since Automatic Maintenance runs strictly on idle-time in its default mode, and stops almost immediately with any user activity, it shouldn’t be causing a problem like this – while on the other hand, the excessive CPU utilization would make perfect sense if you had upgraded without uninstalling MSE, or if the old Windows Defender service somehow wasn’t disabled with the upgrade. But I can’t really tell what’s going on there, because you appear to have leftover scheduled scan tasks from both of those programs.