Speeding up pixelwise operations

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I would like to compute an average LoG (Laplacian of Gauss) score over a given AOI. The problem, with the naive approach is, that it takes a really long time, and i have a stong feeling that it may have been done faster.

cv::Mat src = cv::Mat( imgSize.height(), imgSize.width(), CV_8UC4, (void*)imgPtr, (size_t)pitch );

cv::Mat roi;
if( subROI.isNull() ) {
    roi = src;
} else {
    cv::Rect rect = cv::Rect( subROI.x(), subROI.y(), subROI.width(), subROI.height() );
    roi = src(rect);
}
cv::Mat grey;
cv::cvtColor(roi, grey, cv::COLOR_RGB2GRAY);
cv::GaussianBlur(grey, grey, cv::Size(3, 3), 0, 0, cv::BORDER_DEFAULT);
cv::Mat laplaced;
cv::Laplacian(grey, laplaced, CV_16S, 3, 1, 0, cv::BORDER_DEFAULT);
cv::convertScaleAbs(laplaced, laplaced);

float avg = 0.0;
for (int i = 0; i < laplaced.rows; i++)
{
    for (int j = 0; j < laplaced.cols; j++)
    {
        avg += (float)laplaced.at<unsigned char>(i, j);
    }
}
avg /= laplaced.rows*laplaced.cols;

return avg;

Any tip, to make the code run faster? (gonna run on mobile ivy bridge CPU)