But make it or not system slowly?
You don't have to disable it to keep it from slowing down your system.
I have it set for real-time scanning and it uses about 70 MB of memory on a x64 system. It will be less on a x32 system.
If you are concerned about it using CPU power and slowing down your system, then just go into Windows Defender-+------Tools and unclick the real-time scan button.. Then set it to run a scheduled Quick Scan once a week, preferably when your computer is on but you are not using it. It runs fairly quickly. Also, it will use less memory when the real-time scanning is turned off.
It has occasionally found stuff that avast! has missed.