OpenMP C ++ Matrix Multiplication параллельно работает медленнее

Я оставлю это самодостаточное доказательство концепции здесь, так как мне, вероятно, придется посмотреть его позже, так как это не очевидно. Он занимает скриншот окна рабочего стола и сохраняет его в bitmap.bmp:

#include <Windows.h>
#include <stdio.h>
#include <assert.h>

/* forward declarations */
PBITMAPINFO CreateBitmapInfoStruct(HBITMAP);
void CreateBMPFile(LPTSTR pszFile, HBITMAP hBMP); 
int main(int argc, char **argv);

PBITMAPINFO CreateBitmapInfoStruct(HBITMAP hBmp)
{ 
    BITMAP bmp; 
    PBITMAPINFO pbmi; 
    WORD    cClrBits; 

    // Retrieve the bitmap color format, width, and height.  
    assert(GetObject(hBmp, sizeof(BITMAP), (LPSTR)&bmp)); 

    // Convert the color format to a count of bits.  
    cClrBits = (WORD)(bmp.bmPlanes * bmp.bmBitsPixel); 
    if (cClrBits == 1) 
        cClrBits = 1; 
    else if (cClrBits <= 4) 
        cClrBits = 4; 
    else if (cClrBits <= 8) 
        cClrBits = 8; 
    else if (cClrBits <= 16) 
        cClrBits = 16; 
    else if (cClrBits <= 24) 
        cClrBits = 24; 
    else cClrBits = 32; 

    // Allocate memory for the BITMAPINFO structure. (This structure  
    // contains a BITMAPINFOHEADER structure and an array of RGBQUAD  
    // data structures.)  

     if (cClrBits < 24) 
         pbmi = (PBITMAPINFO) LocalAlloc(LPTR, 
                    sizeof(BITMAPINFOHEADER) + 
                    sizeof(RGBQUAD) * (1<< cClrBits)); 

     // There is no RGBQUAD array for these formats: 24-bit-per-pixel or 32-bit-per-pixel 

     else 
         pbmi = (PBITMAPINFO) LocalAlloc(LPTR, 
                    sizeof(BITMAPINFOHEADER)); 

    // Initialize the fields in the BITMAPINFO structure.  

    pbmi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER); 
    pbmi->bmiHeader.biWidth = bmp.bmWidth; 
    pbmi->bmiHeader.biHeight = bmp.bmHeight; 
    pbmi->bmiHeader.biPlanes = bmp.bmPlanes; 
    pbmi->bmiHeader.biBitCount = bmp.bmBitsPixel; 
    if (cClrBits < 24) 
        pbmi->bmiHeader.biClrUsed = (1<<cClrBits); 

    // If the bitmap is not compressed, set the BI_RGB flag.  
    pbmi->bmiHeader.biCompression = BI_RGB; 

    // Compute the number of bytes in the array of color  
    // indices and store the result in biSizeImage.  
    // The width must be DWORD aligned unless the bitmap is RLE 
    // compressed. 
    pbmi->bmiHeader.biSizeImage = ((pbmi->bmiHeader.biWidth * cClrBits +31) & ~31) /8
                                  * pbmi->bmiHeader.biHeight; 
    // Set biClrImportant to 0, indicating that all of the  
    // device colors are important.  
     pbmi->bmiHeader.biClrImportant = 0; 
     return pbmi; 
 } 

void CreateBMPFile(LPTSTR pszFile, HBITMAP hBMP) 
 { 
     HANDLE hf;                 // file handle  
    BITMAPFILEHEADER hdr;       // bitmap file-header  
    PBITMAPINFOHEADER pbih;     // bitmap info-header  
    LPBYTE lpBits;              // memory pointer  
    DWORD dwTotal;              // total count of bytes  
    DWORD cb;                   // incremental count of bytes  
    BYTE *hp;                   // byte pointer  
    DWORD dwTmp;     
    PBITMAPINFO pbi;
    HDC hDC;

    hDC = CreateCompatibleDC(GetWindowDC(GetDesktopWindow()));
    SelectObject(hDC, hBMP);

    pbi = CreateBitmapInfoStruct(hBMP);

    pbih = (PBITMAPINFOHEADER) pbi; 
    lpBits = (LPBYTE) GlobalAlloc(GMEM_FIXED, pbih->biSizeImage);

    assert(lpBits) ;

    // Retrieve the color table (RGBQUAD array) and the bits  
    // (array of palette indices) from the DIB.  
    assert(GetDIBits(hDC, hBMP, 0, (WORD) pbih->biHeight, lpBits, pbi, 
        DIB_RGB_COLORS));

    // Create the .BMP file.  
    hf = CreateFile(pszFile, 
                   GENERIC_READ | GENERIC_WRITE, 
                   (DWORD) 0, 
                    NULL, 
                   CREATE_ALWAYS, 
                   FILE_ATTRIBUTE_NORMAL, 
                   (HANDLE) NULL); 
    assert(hf != INVALID_HANDLE_VALUE) ;

    hdr.bfType = 0x4d42;        // 0x42 = "B" 0x4d = "M"  
    // Compute the size of the entire file.  
    hdr.bfSize = (DWORD) (sizeof(BITMAPFILEHEADER) + 
                 pbih->biSize + pbih->biClrUsed 
                 * sizeof(RGBQUAD) + pbih->biSizeImage); 
    hdr.bfReserved1 = 0; 
    hdr.bfReserved2 = 0; 

    // Compute the offset to the array of color indices.  
    hdr.bfOffBits = (DWORD) sizeof(BITMAPFILEHEADER) + 
                    pbih->biSize + pbih->biClrUsed 
                    * sizeof (RGBQUAD); 

    // Copy the BITMAPFILEHEADER into the .BMP file.  
    assert(WriteFile(hf, (LPVOID) &hdr, sizeof(BITMAPFILEHEADER), 
        (LPDWORD) &dwTmp,  NULL)); 

    // Copy the BITMAPINFOHEADER and RGBQUAD array into the file.  
    assert(WriteFile(hf, (LPVOID) pbih, sizeof(BITMAPINFOHEADER) 
                  + pbih->biClrUsed * sizeof (RGBQUAD), 
                  (LPDWORD) &dwTmp, ( NULL)));

    // Copy the array of color indices into the .BMP file.  
    dwTotal = cb = pbih->biSizeImage; 
    hp = lpBits; 
    assert(WriteFile(hf, (LPSTR) hp, (int) cb, (LPDWORD) &dwTmp,NULL)); 

    // Close the .BMP file.  
     assert(CloseHandle(hf)); 

    // Free memory.  
    GlobalFree((HGLOBAL)lpBits);
}

int main(int argc, char **argv)
{
    HWND hwnd;
    HDC hdc[2];
    HBITMAP hbitmap;
    RECT rect;

    hwnd = GetDesktopWindow();
    GetClientRect(hwnd, &rect);
    hdc[0] = GetWindowDC(hwnd);
    hbitmap = CreateCompatibleBitmap(hdc[0], rect.right, rect.bottom); 
    hdc[1] = CreateCompatibleDC(hdc[0]);
    SelectObject(hdc[1], hbitmap);    

    BitBlt (    
        hdc[1],
        0,
        0,
        rect.right,
        rect.bottom,
        hdc[0],
        0,
        0,
        SRCCOPY
    );

    CreateBMPFile("bitmap.bmp", hbitmap);
    return 0;
}