Windows Defender has had a history of not playing perfectly nicely with other antivirus installed - they can start arguing over who should scan a file, or if one scans it the other notices and triggers the on access scanner - back and forth for a while wasting resources. To be fair, installing any two virus scanners at the same time with real time or on access scanning will likely give this behaviour. Full system scans could also use a lot of CPU and disk activity (which is why I recommend using only on access scanning).
I've not seen any descriptions of removing Windows Defender entirely. If there are certain files that could be deleted, I don't know them.
When you install another antivirus, Windows Defender should automatically be disabled:
Starting with the Windows 10 Anniversary Update in the Summer of 2016,
Windows Defender can actually run alongside another antivirus app.
When you install another app, Windows Defender itself isn’t
disabled—just its real-time protection component is. That means your
third-party app handles real-time protection, but you can still run a
manual scan with Defender whenever you want.
To disable on access scanning manually, Microsoft says
- Open Windows Defender Security Center, then select Virus & threat
protection > Threat settings.
- Turn off Real-time protection. Note:
Scheduled scans will continue to run. However, files that are
downloaded or installed will not be scanned.
If you find it re-enables itself, methods for keeping it turned off more permanently include:
Open Regedit and browse to
HKEY_LOCAL_MACHINE > SOFTWARE > Policies > Microsoft > Windows Defender
Create or edit a key called 'DisableAntiSpyware' to have the value '1'.
Or, if you have a Pro or Enteprise edition of Windows:
Press the “Windows key + R” to load the Run box, then type gpedit.msc
into the box and press OK to get the group policy editor. Then browse
Computer Configuration > Administrative Templates> Windows
Components > Windows Defender
Open the “Turn off Windows Defender”
policy, and enable it.