PIL: How do I efficiently convert a gray image into black white

I want to have only black or white pixels in a resulting image. I tried the contrast and other filter methods w/o success.

I ended up with the code (pixel access) below which works for me but is slow. I don't know how to implement the code with getdata() and putdata():

for x in range(width):
    for y in range(height):
        pix = imgcic.getpixel((x, y))
        if pix < 200:
            pix = 0
        else:
            pix = 255
        imgcic.putpixel((x,y), pix)

This is the original picture. It has some artefacts on the right top

This is the resulting picture w/o artefacts after deciding black/white based on a brightness threshold