How to optimise large loops for debug mode

I have implemented a pixel mask class used for checking for perfect collision. I am using SFML so the implementation is fairly straight forward:

Loop through each pixel of the image and decide whether its true or false based on its transparency value. Here is the code I have used:

// Create an Image from the given texture
    sf::Image image(texture.copyToImage());

    // measure the time this function takes
    sf::Clock clock;
    sf::Time time = sf::Time::Zero;
    clock.restart();

    // Reserve memory for the pixelMask vector to avoid repeating allocation
    pixelMask.reserve(image.getSize().x);

    // Loop through every pixel of the texture
    for (unsigned int i = 0; i < image.getSize().x; i++)
    {
        // Create the mask for one line
        std::vector<bool> tempMask;
        // Reserve memory for the pixelMask vector to avoid repeating allocation
        tempMask.reserve(image.getSize().y);

        for (unsigned int j = 0; j < image.getSize().y; j++)
        {
            // If the pixel is not transparrent
            if (image.getPixel(i, j).a > 0)
                // Some part of the texture is there --> push back true
                tempMask.push_back(true);
            else
                // The user can't see this part of the texture --> push back false
                tempMask.push_back(false);
        }
        pixelMask.push_back(tempMask);
    }

    time = clock.restart();
    std::cout << std::endl << "The creation of the pixel mask took: " << time.asMicroseconds() << " microseconds (" << time.asSeconds() << ")";

I have used the an instance of the sf::Clock to meassure time.

My problem is that this function takes ages (e.g. 15 seconds) for larger images(e.g. 1280x720). Interestingly, only in debug mode. When compiling the release version the same texture/image only takes 0.1 seconds or less.

I have tried to reduce memory allocations by using the resize() method but it didn't change much. I know that looping through almost 1 million pixels is slow but it should not be 15 seconds slow should it?

Since I want to test my code in debug mode (for obvious reasons) and I don't want to wait 5 min till all the pixel masks have been created, what I am looking for is basically a way to:

• Either optimise the code / have I missed somthing obvious?
• Or get something similar to the release performance in debug mode

Thanks for your help!