I've never had Windows Defender use a noticable amount of CPU. It is, by far, the least intrusive antivirus/anti-malware that I've used (and I've used them all).
Unfortunately, I can only provide my anecdotal experience, but it seems to be in stark contrast to your comment.
Then you are in luck. Also rise up and stop with that anecdotal circle jerk.
The attack surface of Windows is massive, with legacy cruft tucked into every corner. Not saying Defender couldn't be optimized and improved, just that defending the monolith that is Windows is a monumental task.
This has not been my experience at all, can you elaborate?
Can you tell us more? What is the amount?
Is it? You don't want Windows to come with good antivirus?