opencv playing video slow speed - c++

Question

I have a problem with video speed, when I just run a video file in opencv it plays in normal speed.
The problem starts when I apply face and eye detection to the video it plays very slowly. I have tried to change waitKey() values , but the problem still exists.
Why is this happening?
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace std;
using namespace cv;
// Function Headers
void detectAndDisplay(Mat frame);
// Global variables
// Copy this file from opencv/data/haarscascades to target folder
string face_cascade_name = "haarcascade_frontalface_alt.xml";
string eye_cascade_name = "haarcascade_mcs_eyepair_big.xml";
CascadeClassifier face_cascade;
CascadeClassifier eye_cascade;
string window_name = "Capture - Face detection";
int filenumber; // Number of file to be saved
string filename;
int main(void)
{
VideoCapture capture("m.mp4");
if (!capture.isOpened()) // check if we succeeded
return -1;
// Load the cascade
if (!face_cascade.load(face_cascade_name))
{
printf("--(!)Error loading\n");
return (-1);
}
if (!eye_cascade.load(eye_cascade_name))
{
printf("--(!)Error loading\n eye ");
return (-1);
}
// Read the video stream
Mat frame;
for (;;)
{
capture >> frame;
// Apply the classifier to the frame
if (!frame.empty())
{
detectAndDisplay(frame);
}
else
{
printf(" --(!) No captured frame -- Break!");
break;
}
waitKey(40);
return 0;
}
}
void detectAndDisplay(Mat frame)
{
std::vector<Rect> faces;
std::vector<Rect> eyes;
Mat frame_gray;
Mat crop;
Mat res;
Mat gray;
string text;
stringstream sstm;
cvtColor(frame, frame_gray, COLOR_BGR2GRAY);
equalizeHist(frame_gray, frame_gray);
// Detect faces
face_cascade.detectMultiScale(frame_gray, faces, 1.1, 4, CASCADE_SCALE_IMAGE, Size(20, 20));
// Set Region of Interest
size_t i = 0; // ic is index of current element
for (i = 0; i < faces.size(); i++) // Iterate through all current elements (detected faces)
{
Point pt1(faces[i].x, faces[i].y); // Display detected faces on main window - live stream from camera
Point pt2((faces[i].x + faces[i].height), (faces[i].y + faces[i].width));
rectangle(frame, pt1, pt2, Scalar(0, 255, 0), 2, 8, 0);
// set ROI for the eyes
Rect Roi = faces[i];
Roi.height = Roi.height / 4;
Roi.y = Roi.y + Roi.height;
cv::Mat crop = frame(Roi);
cvtColor(crop, gray, CV_BGR2GRAY);
imshow("ROI", gray);
eye_cascade.detectMultiScale(gray, eyes, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(20, 20));
for (size_t j = 0; j < eyes.size(); j++)
{
Rect r(Roi.x + eyes[j].x, Roi.y + eyes[j].y, eyes[j].width, eyes[j].height);
rectangle(frame, r, Scalar(255, 0, 0), 3, 1, 0);
// convert roi to ychannel
Mat eye_region = frame(eyes[j]).clone();
Mat eye_region_YCbCr;
cvtColor(eye_region, eye_region_YCbCr, CV_BGR2YCrCb);
vector<Mat> channels;
split(eye_region_YCbCr, channels);
Mat eye_region_Ychannel = channels[0].clone();
imshow("Y", eye_region_Ychannel);
// end of convert
}
}
// Show image
imshow("original", frame);
if (!crop.empty())
{
imshow("detected", crop);
}
else
{
destroyWindow("detected");
}
}