opencv much slower in multithreading

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Im writting a console application that uses open cv and multithreading. Im testing it in a 4 native core CPU (8 with HT enabled) with 12 GB of RAM. Each thread has to execute a function that uses opencv calls. In that case, the time is much bigger executing in parallel several threads than the run time obtained for a single thread. One expects that the time is more or less the same independently the number of threads or increased about 10%, but the run time for each thread increases according to the number of threads used. The more threads, the more the time for each one.

I have test cv::setnumThreads(8) and cv::setnumThreads(0) with the same result. If the function is replaced by other function with some own dummy filters over data, the behaviour is the expected one, all threads ends with the same run time independently of the numbre of threads. Does opencv functions block the threads or do some sequential operations that blok threads???. The picture below shows the times obtained in the application:

Time in file No. 3 --> means the total time to process a sequence of images (768) in miliseconds,

I attach a sample c++ project to test this behaviour. This application executes secuentially 1 to 8 threads. Does anybody know what is happening?. I don`t know what else to do...

Thanks.

EDIT. Here is the code. I cannot attach a ready to use project to the post. It is in the link of the previous comment. The test image is Test.jpg. The original used is in bmp format.

#include "stdafx.h"
#include <future>
#include <chrono>
#include "Filter.h"
#include <iostream>
#include <future>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

long long Ticks();
int WithOpencv(cv::Mat img);
int With_OUT_Opencv(cv::Mat img);
int TestThreads (char *buffer,std::string file);
#define Blur3x3(matrix,f,c) ((matrix[(f-1)*1600+(c-1)] + matrix[(f-1)*1600+c] + matrix[(f-1)*1600+(c+1)] + matrix[f*1600+(c-1)] + matrix[f*1600+c] + matrix[f*1600+(c+1)] + matrix[(f+1)*1600+(c-1)] + matrix[(f+1)*1600+c] + matrix[(f+1)*1600+(c+1)])/9)


int _tmain(int argc, _TCHAR* argv[])
{

    std::string file="Test.bmp";

    auto function = [&](char *buffer){return TestThreads(buffer,file);};
    char *buffers[12];
    std::future<int> frames[12];
    DWORD tid;
    int i,j;
    int nframes = 0;
    int ncores;

    cv::setNumThreads(8);

    for (i=0;i<8;i++) buffers[i] = new char[1000*1024*1024];
    for (j=1;j<9;j++)
    {
        ncores = j;
        long long t = Ticks();
        for (i=0;i<ncores;i++) frames[i] = std::async(std::launch::async,function,buffers[i]);
        for (i=0;i<ncores;i++) nframes += frames[i].get();
        t = Ticks() - t;

        std::cout << "Mean time using " << ncores << " cores is: " << t/nframes << "ms" << std::endl << std::endl;
        nframes = 0;
        Sleep(2000);
    }
    for (int i=0;i<8;i++) delete buffers[i];

    return NULL;

    return 0;
}


int TestThreads (char *buffer,std::string file)
{

    long long ta;
    int res ...