How do i publish very fast?

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Hi! I am working with the ar drone and ROS. Im trying to make it track a ball. The problem that Im having is that the rate at which I publish message to cmd_vel topic is too slow. However, the rate at which i show the command in the terminal is normal (fast), so i know that it is not problem of the algorithm itself, but the publishing of messages. Here is my code:

#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <sensor_msgs/image_encodings.h>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <cv_bridge/cv_bridge.h>
#include <geometry_msgs/Twist.h>
#include <std_msgs/Empty.h>
#include <std_msgs/String.h>
#include <sstream>
using namespace cv;
//Values for Trackbar, Hue, Saturation Value
int iLowH=150;
int iHighH=179;
int iLowS=87;
int iHighS=255;
int iLowV=71;
int iHighV=152;

int posX,posY;
float ex,ey;

geometry_msgs::Twist twist_msg;
std_msgs::Empty emp_msg;

class BallTracking
{

public:

    void imageCallback(const sensor_msgs::ImageConstPtr& msg)
    {
        ros::Rate loop_rate(50);
        int count=0;
        Point pt;
        ros::NodeHandle nh;
        ros::NodeHandle neu;
        ros::Publisher pub;
        ros::Publisher pub_empty_land;
        ros::Publisher pub_twist;
        ros::Publisher chatter_pub=neu.advertise<std_msgs::String>("chatter", 1000);
        geometry_msgs::Twist twist_msg_hover;
        pub_empty_land = neu.advertise<std_msgs::Empty>("/ardrone/land", 1); /* Message queue length is just 1 */
        pub = neu.advertise<geometry_msgs::Twist>("/cmd_vel", 1);

        //Communicating between ROS an OpenCV
        cv_bridge::CvImagePtr cv_ptr;

        cv::namedWindow("Control", CV_WINDOW_AUTOSIZE); 

        cv::Mat img_thr;

          try
          {
            cv_ptr=cv_bridge::toCvCopy(msg, sensor_msgs::image_encodings::BGR8);
            cvCreateTrackbar("LowH", "Control", &iLowH, 179); 
            cvCreateTrackbar("HighH", "Control", &iHighH, 179); 
            cvCreateTrackbar("LowS", "Control", &iLowS, 255); 
            cvCreateTrackbar("HighS", "Control", &iHighS, 255); 
            cvCreateTrackbar("LowV", "Control", &iLowV, 255); 
            cvCreateTrackbar("HighV", "Control", &iHighV, 255);
            //Thresholding
            cv::cvtColor(cv_ptr->image,img_thr,CV_BGR2HSV); 
            cv::inRange(img_thr, cv::Scalar(iLowH,iLowS,iLowV), cv::Scalar(iHighH,iHighS,iHighV), img_thr); 
            //Voilá
            Moments oMoments = moments(img_thr);

            double dM01 = oMoments.m01;
            double dM10 = oMoments.m10;
            double dArea = oMoments.m00;

            if (dArea > 10000)
            {
                //calculate the position of the ball
                 posX = dM10 / dArea;
                 posY = dM01 / dArea;
                pt.x=posX;
                pt.y=posY;  
                circle(cv_ptr->image, pt, 50, Scalar(0,0,255), 1, 8, 0);
                ex=(319-posX)/319.0;
                ey=(179-posY)/179.0;
                ROS_INFO("Ball Position:[%f,%f]",ex,ey);
                twist_msg.linear.y=ex;
                //ROS_INFO("%d", msg.data);                         
                pub.publish(twist_msg);
                ros::spinOnce();



            }
            else
            {
                posX=319;
                posY=179;
                ROS_INFO("Landing!!!!");
                pt.x=posX;
                pt.y=posY;
                ex=(319-posX)/319.0;
                ey=(179-posY)/179.0;  
                circle(cv_ptr->image, pt, 50, Scalar(0,0,255), 1, 8, 0);
                std_msgs::String msg;
                std::stringstream ss;
                ss<< "Holi world" << count;
                msg.data==ss.str();
                //chatter_pub.publish(msg);
                //pub_empty_land.publish(emp_msg);
            }


            cv::imshow("view",img_thr);
            cv::imshow("Control",cv_ptr->image);
            cv::waitKey(30);
          }
          catch (cv_bridge::Exception& e)
          {
            ROS_ERROR("Could not convert from '%s' to 'bgr8'.", msg->encoding.c_str());
          }

    ros::spinOnce();
    }
};

int main(int argc, char **argv)
{
    //Initializing ROS
    ros::init(argc, argv, "tracker");
    ros::NodeHandle nh;
    ros::Rate loop_rate(50);    
    cv::namedWindow("view");
    cv::startWindowThread();
    BallTracking bt;
    twist_msg.linear.x=0.0;
    twist_msg.linear.y=0.0;
    twist_msg.angular.z=0.0;
    twist_msg.angular.x=0.0 ...