MSS (written in pure python, working on windows, mac, linux) is capable to grab the screen at ~40 fps at 800x400 resolution.

screen_grabber.py

import numpy as np
import cv2
import time
from mss import mss
from PIL import Image

'''Macbook have a bug about box's dimensions, can be just multiple of 16
but this bug will be fix soon in the next release'''
box = { "top": 10, "left": 10, "width": 400, "height": 320}

sct = mss()

while True:

    t = time.time()
    sct.get_pixels(box)
    image = Image.frombytes('RGB', (sct.width, sct.height), sct.image)

    # Could be useful in MAC OS    
    #b, g, r = image.split()
    #image = Image.merge("RGB", (r, g, b))

    cv2.imshow('test', np.array(image))
    print('fps: {}'.format(1/(time.time()-t)))

    if cv2.waitKey(25) & 0xFF == ord('q'):
        cv2.destroyAllWindows()
        break