Efficient matrix operator

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Hi all, I use mat format to calculate an matrix, also i use traditions C++ calculate an matix. But I find out, mat format is very slowly.Could someone tell me reason ? And how to improve the efficiency. Any help is highly appreciated! Thanks:) Here is my code:

1. traditions method where sw=src2.step,it almost costs 5 number of ticks/one cycle.

    for (int k=0;k<100;k++)
    {
       int A = getTickCount();  
       for (int iwr = 0; iwr <= 11; iwr++ )
           {
             /* Sub-Window 0 */
             for (int iwc =0; iwc <= 11; iwc++ )
               {
                gray_val =src2.data[iwr*sw+iwc];
                sum[0] += gray_val;
                sum_sq[0] += gray_val * gray_val;
                }
   
             /* Sub-Window 1 */
          for (int iwc = 0; iwc &lt;=11; iwc++ )
           {
            gray_val = src2.data[iwr*sw+iwc];
            sum[1] += gray_val;
            sum_sq[1] += gray_val * gray_val;
            }
           }
         int B = getTickCount(); 
         int c=B-A;
   
   
    }
   

2.it almost costs 377 number of ticks/one cycle.

for(int k=0;k<100;k++)
{
            int A = getTickCount(); 
            Mat ROIimage1(src2, Rect(0,0,11,11) );
            sum1=cv::sum(ROIimage1);
            cv::multiply(ROIimage1,ROIimage1,square1);
            sum_sq1=cv::sum(square1);
        Mat ROIimage2(src2, Rect(0,0,11,11) );//=src2(ROI2);
        sum2=cv::sum(ROIimage2);
        cv::multiply(ROIimage2,ROIimage2,square2);
        sum_sq2=cv::sum(square2);
       int B = getTickCount(); 
        int c=B-A;
}