why there is a pause selecting UIImage from PhotoLibrary?

Question

the code is as follows:

-(void)imagePickerController:(UIImagePickerController*)picker didFinishPickingMediaWithInfo:(NSDictionary*) info{

   UIImage* pickedImg=[info objectForKey:UIImagePickerControllerOriginalImage];
   imageView.image   =pickedImg;//imageView is set to "aspect fit".
}

if the sourceType is from camera,after selecting the image,the imageview will display the picking image soon.but if the sourceType is from album,after touch the thumbnail,the user has to wait for a second before switching to the imageView. However,iPhone builit-in Photos doesn't have such kind of problem. Why? How to solve the problem?

Thanks a lot !