Round ImageView with blur when use Kingfisher3.1

I want to make a roundCorner for userAvatarImageView. At first, I used the common ways to UIImageView:

extension UIImageView {
    func makeRoundedCorners(_ radius: CGFloat) {
         self.layer.cornerRadius = radius
         self.layer.masksToBounds = true
   }
}

It was works for me. But I don’t think this is a good way for tableViewCell. So, I used RoundCornerImageProcessor:

let roundProcessor = RoundCornerImageProcessor(cornerRadius: 15,
                                               targetSize: CGSize(width: 30, height: 30))

userAvatarImageView.kf.setImage(with: URL(string: string),
                                    placeholder: UIImage(named: "placeholder"),
                                    options: [ .processor(roundProcessor)])

It can get a roundCorner, but the image was blur, not clearly. like this:

How can I handle this issue?