OpenCV very slow - webcam

I'm using OpenCV to send camera bytes via UDP to another computer.

The problem is, that the framerate of the camera is only 15fps. If I send a picture, it works with over 200fps.

My camera supports 30fps, so does anyone know why this happen?

Here's my code:

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <fstream>
#include <arpa/inet.h>

#include <unistd.h>

using namespace std;
using namespace cv;

void createSocket(int port, const char* ip);
void sendData(vector<uchar> buff);
int _socket;

struct sockaddr_in serverAdress,clientAdress;


// argv[1] = Bild
int main(int argc, char** argv)
{

    if(argc != 5)
    {
        cout << "Error\n";
        return -1;
    }

    createSocket(atoi(argv[2]),argv[1]);

    vector<uchar> buff;
    vector<int> param = vector<int>(2);


    param[0] = CV_IMWRITE_JPEG_QUALITY;
    param[1] = atoi(argv[3]);


    // VideoCapture
    VideoCapture cap(1);
    cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
    cap.set(CV_CAP_PROP_FRAME_HEIGHT, 360);


    if(!cap.isOpened())
    {
        cout << "Error\n";
        return -1;
    }


    Mat frame;

    while(true){

        cap >> frame;

        cvtColor(frame, frame, CV_8U);

        imencode(".jpeg",frame, buff,param);
        cout<<"coded file size(jpg)"<<buff.size()<<endl;
        sendData(buff);
    }


    return 0;
}


void createSocket(int port, const char* ip){
    _socket = socket(AF_INET,SOCK_DGRAM,0);

    serverAdress.sin_family = AF_INET;
    serverAdress.sin_addr.s_addr= inet_addr(ip);
    serverAdress.sin_port=htons(port);

    if(_socket == -1){
        cout << "Error";
        return;
    }

}


void sendData(vector<uchar> buff){

    char data[buff.size()];
    for(int i = 0; i < buff.size(); i++){
        data[i] = buff.at(i);
    }

    sendto(_socket,data,buff.size(),0,(struct sockaddr *)&serverAdress,sizeof(serverAdress));
}