qt working very slow on windows - c++

Question

I am working on opencv and qt project.it 's working very quickly on linux but it is working very slow on windows.Linux OS is 64 bit ,windows 64 bit too.I didnt understand the problem.Thanks (sorry for my english)
following code:
#include <opencv2/core/base.hpp>
#include <opencv2/core/mat.hpp>
#include <opencv2/core/types.hpp>
#include <opencv2/highgui/highgui_c.h>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/objdetect/objdetect_c.h>
#include <opencv2/objdetect.hpp>
#include <opencv2/videoio.hpp>
#include <iostream>
#include <vector>
using namespace cv;
using namespace std;
int main(int argc, char* argv[])
{
VideoCapture cap(0); // open the video camera no. 0
namedWindow("MyVideo",CV_WINDOW_AUTOSIZE); //create a window called "MyVideo"
// Load Face cascade (.xml file)
CascadeClassifier face_cascade;
face_cascade.load( "xml/haarcascade_frontalface_alt.xml" );
while (1)
{
Mat frame;
bool bSuccess = cap.read(frame); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
break;
}
// Detect faces
std::vector<Rect> faces;
face_cascade.detectMultiScale( frame, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );
// Draw circles on the detected faces
for( int i = 0; i < faces.size(); i++ )
{
Point center( faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5 );
ellipse( frame, center, Size( faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );
imshow( "Detected Face", frame );
}
//rectangle( frame, center, Size( faces[i].width*2, faces[i].height*2), Scalar( 255, 0, 255 ) );
if (waitKey(30) == 27) //wait for 'esc' key press for 30ms. If 'esc' key is pressed, break loop
{
cout << "esc key is pressed by user" << endl;
break;
}
}
return 0;
}