Метод прямоугольника не параллелится

Здравствуйте пишу простой алгоритм численного метода (прямоугольника). Распараллелил эту функцию с помощью OpenMP. Но время выполнения параллельного алгоритма больше, чем выполнение с одним поток. В чем ошибка? Использую пк с 12 ядрами процессора,999999999 итераций.

const unsigned long numSteps=999999999;                     /* default # of rectangles */
const double PI25DT = 3.141592653589793238462643;
double function(double x) //Подынтегральная функция
{
  return 4.0/(1.+ x*x); //Например, sin(x)
}
double CalcIntegralWithParallel(int countThread) {
   double x, pi = 0;
   double sum=0.0;
   double step = 1./static_cast<double>(numSteps);
 #pragma omp parallel for num_threads(countThread) reduction(+:sum)
    for (int i=0; i<numSteps; i++)
    {
        x = (i + .5)*step;
        sum = sum + function(x);
    }
    pi = sum*step;
    return pi;
}
double CalcIntegral(double a, double b) {
    double x, pi = 0;
    double sum=0.0;
    double step =1./static_cast<double>(numSteps);
    for (int i=0; i<numSteps; i++)
    {
        x = (i + .5)*step;
        sum = sum + function(x);
    }
    pi = sum*step;
    return pi;
}
int main(int argc, char** argv)
{
    double timeStart ,timeStop;
    double pi; 
    double secs;
    timeStart = omp_get_wtime();
    pi = CalcIntegral(8,0);
    timeStop = omp_get_wtime();
    std::cout << "The value of integral is " << pi << " Error is " << fabs(pi - PI25DT) << std::endl;
    std::cout << "The time to calculate integral was " ;
    secs=  timeStop - timeStart;
    std::cout << secs << " seconds\n" << std::endl;
    timeStart = omp_get_wtime();
    pi = CalcIntegralWithParallel(4);
    timeStop = omp_get_wtime();
    std::cout << "The value of PI is OPEN MP 4 " << pi << " Error is " << fabs(pi - PI25DT) << std::endl;
    std::cout << "The time to calculate PI was " ;
    secs=  timeStop - timeStart;
    std::cout << secs << " seconds\n" << std::endl;
    timeStart = omp_get_wtime();
    pi = CalcIntegralWithParallel(8);
    timeStop = omp_get_wtime();
    std::cout << "The value of PI is OPEN MP 8 " << pi << " Error is " << fabs(pi - PI25DT) << std::endl;
    std::cout << "The time to calculate PI was " ;
     secs=  timeStop - timeStart;
    std::cout << secs << " seconds\n" << std::endl;
    timeStart = omp_get_wtime();
    pi = CalcIntegralWithParallel(12);
    timeStop = omp_get_wtime();
    std::cout << "The value of PI is OPEN MP 12 " << pi << " Error is " << fabs(pi - PI25DT) << std::endl;
    std::cout << "The time to calculate PI was " ;
     secs=  timeStop - timeStart;
    std::cout << secs << " seconds\n" << std::endl;
    return 0;
}

Ниже результат выполнения программы:

The value of integral is 3.14159 Error is 7.54952e-15 The time to calculate integral was 11.9711 seconds

The value of PI is OPEN MP 4 3.14162 Error is 2.64888e-05 The time to calculate PI was 20.342 seconds

The value of PI is OPEN MP 8 3.14159 Error is 3.7927e-07 The time to calculate PI was 21.1447 seconds

The value of PI is OPEN MP 12 3.14162 Error is 3.16601e-05 The time to calculate PI was 21.4869 seconds