C# - Bitmap schneller in Graustufen wandeln

private Bitmap BildSW(Bitmap bmp)
{
    int numBytes = bmpData.Stride * bmp.Height;
    byte[] rgbValues = new byte[numBytes];
    int tmpSW;
    
    PixelFormat pxf = PixelFormat.Format24bppRgb;
	
    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, pxf);
	
    IntPtr ptr = bmpData.Scan0;
	
    Marshal.Copy(ptr, rgbValues, 0, numBytes);
	
    for(int counter = 0; counter < rgbValues.Length; counter+=3)
    {
        tmpSW  = rgbValues[counter];
        tmpSW += rgbValues[counter+1];
        tmpSW += rgbValues[counter+2];
        
    	tmpSW /= 3;
    	
    	rgbValues[counter] = rgbValues[counter+1] = rgbValues[counter+2] = Convert.ToByte(tmpSW);
    }
    
    Marshal.Copy(rgbValues, 0, ptr, numBytes);
	
    bmp.UnlockBits(bmpData);

    return bmp;
}