I'm looking for a way to speed up the drawing of my game engine, which is currently the significant bottleneck, and is causing slowdowns. I'm on the verge of converting it over to XNA, but I just noticed something.

Say I have a small image that I've loaded.

    Image img = Image.FromFile("mypict.png");

We have a picturebox on the screen we want to draw on. So we have a handler.

    pictureBox1.Paint += new PaintEventHandler(pictureBox1_Paint);

I want our loaded image to be tiled on the picturebox (this is for a game, after all). Why on earth is this code:

    void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        for (int y = 0; y < 16; y++)
            for (int x = 0; x < 16; x++)
                e.Graphics.DrawImage(image, x * 16, y * 16, 16, 16);
    }

over 25 TIMES FASTER than this code:

    Image buff = new Bitmap(256, 256, PixelFormat.Format32bppPArgb); // actually a form member
    void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        using (Graphics g = Graphics.FromImage(buff))
        {
            for (int y = 0; y < 16; y++)
                for (int x = 0; x < 16; x++)
                    g.DrawImage(image, x * 16, y * 16, 16, 16);
        }
        e.Graphics.DrawImage(buff, 0, 0, 256, 256);
    }

To eliminate the obvious, I've tried commenting out the last e.Graphics.DrawImage (which means I don't see anything, but it gets rid a call that isn't in the first example). I've also left in the using block (needlessly) in the first example, but it's still just as blazingly fast. I've set properties of g to match e.Graphics - things like InterpolationMode, CompositingQuality, etc, but nothing I do bridges this incredible gap in performance. I can't find any difference between the two Graphics objects. What gives?

My test with a System.Diagnostics.Stopwatch says that the first code snippet runs at about 7100 fps, while the second runs at a measly 280 fps. My reference image is VS2010ImageLibrary\Objects\png_format\WinVista\SecurityLock.png, which is 48x48 px, and which I modified to be 72 dpi instead of 96, but those made no difference either.