Take a screenshot of the area you're interested in and read the pixels from that bitmap. Then access the pixels from the scanlines property of the bitmap if possible. Should be a lot faster.
GetPixel is generally not a fast function. And some Windows 8 screen dpi changes can also slow down graphics quite a lot. So it is hard to tell what is slowing down your code. You'd have to profile your code if you want to know which part is so much slower on this Win8 PC.