With every such question, you mostly get 2 kinds of answers. Subjectives and Objectives.
Subjective because each PC, each user, and each use scenario is different.
* A lot depends on your user's behavior. Do you download from trusted sources or whatever you find? Do you open every email you see? Half the protection starts from knowing what to do but most important, what NOT to do.
* It depends on PC specs up to a point. Let's say an a/v uses 1 core to do a scan. If you have a dual-core that means it uses 50% of your CPU, if you have a 6-core it uses 16% of your CPU and if you have a 12-core it uses only 8%... So is it slow at 50% or fast at 8%???
* Do you download and update a lot of files like more than 500 a day?? A/v will take more resources than if you download a program every week. A/V doesn't rescan things they already did unless there is a change.
* Do you use SSD, NVMe, or old spinner disks? Performance depends a LOT on this factor alone.
So If one uses a dual-core with a spinner disk, any antivirus will suck and slow the PC down a lot, if I use a 6core and SSD, you probably won't even feel it. That's why opinions are subjective.
Objective answers are the answers you can get from independent labs (avoid the mainstream "tests" as those are paid advertisements) and see statistics and compare on your own. For example
https://www.av-test.org/en/antivirus/home-windows/
The image speaks for itself
Comparing apples to apples.
Defender is NOT an antivirus only. It protects Windows files and your files from ransomware by restricting access to document folders and so you will never see your files renamed. It offers account protection (offline and online), antitamper protection, and many more while other antivirus offers less. My point tho here is when you have an APP that does 10 different things, it's not fair to compare it with an app that does 2 of those, even tho they are both antiviruses and you need to weigh this in.
It's like asking 1 person to do 10 things, another to do 2 things, and then wondering why the person that does 2 things finishes faster.
So my suggestion and subjective answer based on personal experience on my own PCs and clients' PCs is that Defender is an excellent program but you should check and form your own opinion. After all, why not test-drive it? Install a Windows with Defender, set it to which folders to scan and which not, let a couple of days pass, and see how your life is. If you don't like it, remove it with one of the many tools on the net.