It eats up a lot of CPU. It doesn’t seem like much help in a default update enabled system where you are using a regular user account instead of an administrator account.
In addition, anti-virus and real time scanning is itself potential surface area for an exploit (for example a few years back there was an exploit based on Norton antivirus email scanner).
If you have a Pro version of Windows there is a group policy setting for it. [1]
If you have Home, you can achieve the same effect by manually tweaking the registry. [2]
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[1] Computer Configuration > Administrative Templates > Windows Components > Windows Defender Antivirus > Real-time Protection
[2] HKLM\SOFTWARE\Policies\Microsoft\Windows Defender\Real-Time Protection\"DisableRealtimeMonitoring"=dword:00000001