最常用的设置圆角的方法：

view.layer.cornerRadius = 5
view.layer.masksToBounds = true

但是masksToBounds这个属性会造成离屏渲染，这个属性才是帧数下降的罪魁祸首。

如果想简便的话可以设置

view.layer.shouldRasterize=true

这个属性可以为圆角设置缓存，但是设置缓存也是需要时间的，如果有大量的大小不一的圆形视图出现，这样做依然会出现掉帧。

下面提供两种思路解决当页面上需要设置的视图较多时的解决方案：
一、通过绘制方法，将view的背景图片重新去掉圆角绘制一遍
1.UIView。注:(该方法只返回UIView类型，其他类型会出问题)

extension UIView {
    func circleView(cornerRadius: CGFloat) -> UIView? {
        // 开始图形上下文
        UIGraphicsBeginImageContextWithOptions(self.frame.size, false, 0)
        if let context = UIGraphicsGetCurrentContext() {
            let rect = CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)
            var path: UIBezierPath
            if let color = self.backgroundColor {
                CGContextSetLineWidth(context, 1)
                CGContextSetFillColorWithColor(context, color.CGColor)
                CGContextSetStrokeColorWithColor(context, color.CGColor)
            }
            
            if self.frame.size.width == self.frame.size.height {
                if cornerRadius == self.frame.size.width/2 {
                    path = UIBezierPath(arcCenter: self.center, radius: cornerRadius, startAngle: 0, endAngle: 2.0*CGFloat(M_PI), clockwise: true)
                }else {
                    path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
                }
            }else {
                path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
            }
            self.drawRect(rect)
            CGContextAddPath(context, path.CGPath)
            CGContextDrawPath(context, .FillStroke)
            CGContextClip(context)
            // 从上下文上获取剪裁后的照片
            guard let uncompressedImage = UIGraphicsGetImageFromCurrentImageContext() else {
                UIGraphicsEndImageContext()
                return nil
            }
            // 关闭上下文
            UIGraphicsEndImageContext()
            let view = UIView()
            view.backgroundColor = UIColor.clearColor()
            view.frame = self.frame
            view.addSubview(UIImageView(image: uncompressedImage))
            return view
        }else {
            return nil
        }
    }
}

2、UIImage(常用方法)

extension UIImage {
    func circleImage(cornerRadius: CGFloat, size: CGSize) -> UIImage? {
        let rect = CGRectMake(0, 0, size.width, size.height)
        UIGraphicsBeginImageContextWithOptions(size, false, 0)
        if let context = UIGraphicsGetCurrentContext() {
            var path: UIBezierPath
            if size.height == size.width {
                if cornerRadius == size.width/2 {
                    path = UIBezierPath(arcCenter: CGPointMake(size.width/2, size.height/2), radius: cornerRadius, startAngle: 0, endAngle: 2.0*CGFloat(M_PI), clockwise: true)
                }else {
                    path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
                }
            }else {
                path = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius)
            }
            CGContextAddPath(context, path.CGPath)
            CGContextClip(context)
            self.drawInRect(rect)
            // 从上下文上获取剪裁后的照片
            guard let uncompressedImage = UIGraphicsGetImageFromCurrentImageContext() else {
                UIGraphicsEndImageContext()
                return nil
            }
            // 关闭上下文
            UIGraphicsEndImageContext()
            return uncompressedImage
        }else {
            return nil
        }
    }
}

二、通过设置layer，可以设置切割哪个角，且设置边框

let path = UIBezierPath.init(roundedRect: self.myLabel.bounds, byRoundingCorners: [.topRight , .bottomRight] , cornerRadii: self.myLabel.bounds.size);
let layer = CAShapeLayer.init();
layer.path = path.cgPath;
layer.lineWidth = 5;
layer.lineCap = kCALineCapSquare;
layer.strokeColor = UIColor.red.cgColor;
//  注意直接填充layer的颜色，不需要设置控件view的backgroundColor
layer.fillColor = UIColor.yellow.cgColor;
myLabel.layer.addSublayer(layer);