Overview:

Pretty simple, having Windows Defender running in the background VS off. 
Often you will hear something like “no this does NOT make a difference at all, you are just
risking getting a virus!!”

Well i am here to tell you that it does, infact, make a huge difference.
We are talking about a CPU-Upgrade level of performance increase.

Defender ON:

Defender OFF:

Conclusion:

As you can see Defender alone tanks around 10~% CPU performance by itself. 
Even while gaming!!

Now, do i recommend everyone to get rid of Defender?
Absolutely not.

This should only be done by experts who know exactly what they download and install. 

Quick side info, i have been fine without Defender for over 4 years now. So, as mentioned, it really comes down to the individual person.

-Tqhr