Function execution is slow in callbacks. Why?

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Hi My (Heavy) Image processing functionis slow in the subscriber callback. But if I run the same function outside ROS in release mode it is fast. (10X fast)

Why I want to put the image processing function in callback? Because I want to process received image as soon it arrives.

Any comments! My code looks like:

class storedData {
  public:    
    cv::Mat inIm = cv::Mat::zeros(376, 672, CV_8UC3);
    cv::Mat outIm = cv::Mat::zeros(376, 672, CV_8UC3);

    void depthCallback(const sensor_msgs::CompressedImageConstPtr& msgDepth)
    {
      try
      {
        tic2 = ros::Time::now();

        imProcess(inIm, outIm);         // Heavy Image Processing

        imPTime = ros::Time::now().toSec() - tic2.toSec();

        std::cout<<"Im process time= "<<imPTime<<std::endl;

        cv::imshow("view", outIm);
        cv::waitKey(1);
        }
      catch (cv_bridge::Exception& e)
      {
            ROS_ERROR("Could not convert the depth!");
      }
    }
};
int main(int argc, char **argv)
{
  ros::init(argc, argv, "PredictiveDisplay");
  ros::NodeHandle nh;
  storedData obj;
  cv::namedWindow("view", 0);
  cv::startWindowThread();

  ros::Subscriber subDepth = nh.subscribe("/depth/depth_registered/compressed", 2, &storedData::depthCallback, &obj);
  ros::spin();

  cv::destroyWindow("view");
}