在iOS上使用CIFilter创建图像会导致渲染速度非常慢

I use CIFilter to create a UIImage then add it on a UIImageView. The process of creating this image is really fast and I can add it on the image view. But the whole UI is freeze for a few seconds until it shows the filtered image. I checked that the CIFilter call is fast. I think the slow is caused by UIImageView to render the image. Why is it such slow if the image already created? Below is the code to make a filter of a image.I use CIFilter to create a UIImage then add it