Windows Defender (on windows 10) using too much CPU

Can you run following two command through elevated PowerShell and post the output (Number in bold)  

PS C:> $Preferences = Get-MpPreference

PS C:> $Preferences.ScanAvgCPULoadFactor

20

PowerShell can be run using following command

1. Type PowerShell in Cortana search box
2. Right Click on Windows PowerShell and run as administrator 

The answer is 50

I just looked this up and its seems this is the default. I also found you can set it with a registry key and turned it down to 10 ie 10%. 50% is NOT a sensible default IMHO as its just one of the many OS background tasks that the OS runs, and explains why there is so little CPU time left for the poor user.  I will report if this seems to fix the problem but if there are any other tweaks like this then please publish them here.  I also have a laptop on Windows 8.1 and lose it for the first few hours of use every time I turn it on as I use it rarely and it goes off an updates itself.  The record was over 8 hours before I could successfully type something on the laptop as there were over 100 updates to install.....

Open Defender and run a scan -- try Quick first.

The CPU usage may be Defender cataloging files to aid in real time protection prior to the run of a scan. This activity usually happens in the background and is not noticeable except for drive activity.

Sorry, I meant that there were no Windows updates.  The virus definitions were updated before the scan started.

A1b: No, I took no actions when upgrading except for clicking on install.  To my knowledge Windows Defender was still disabled at the point of install.

A4: I stopped saving backups for CCleaners registry cleaning as I trusted it to do a god job, so No, I do not have them all.

I looked up your note [1].  All I can say is that when Windows 8.1 installer COMPLETELY TRASHED may daughters PC CCleaner was the only thing that brought it back to life.  I completely agree that in the real world registry cleaners should not be necessary but if even Microsoft cannot keep control the registry then perhaps the registry was a bad idea in the first place and is an accident waiting to happen.  I'm an engineer. Even so,  I don't want to know anything about the registry. Unfortunately I've had to open it and manually edit it so many times I've lost count.  Even then I've reinstalled OSs more times than I can count too as after a year or so after they are installed they simply stop working.  I've been working with PCs since 1982 and they never got any easier to manage in all the time I've used them. Actually that is not completely true.  Windows 7 is a pretty good OS and I think I only reinstalled my main PC twice since I first installed Windows 7.  I put all my important data on A (Linux based) NAS and never on my C: drive as I never know when I will need to reinstall Windows.  My brothers (both Software engineers) say I should use Linux but I like to play computer games, so I stick with Windows. Still this is all philosophical and not the point of this thread.  The PC in question is working well at this moment, playing Youtube tracks from the film "Annie" so for me this thread is done.

Can you run following two command through elevated PowerShell and post the output (Number in bold)  

PS C:> $Preferences = Get-MpPreference

PS C:> $Preferences.ScanAvgCPULoadFactor

20

PowerShell can be run using following command

1. Type PowerShell in Cortana search box
2. Right Click on Windows PowerShell and run as administrator

Are there any other antivirus, antispyware, or other security products installed, even if disabled. Were other antivirus products ever on the PC, including trial versions and if so, how were they removed?

You may need to use a cleanup tool to remove your prior security software. See this Wiki article: List of anti-malware product removal tools

What was your prior operating system -- that is did you upgrade from Windows 7 or 8.x?

Was this issue evident in that version?

-steve