With all of the above solutions, I was unable to get a usable frame rate until I modified my code in the following way:
import numpy as np
import cv2
from mss import mss
from PIL import Image
bounding_box = {'top': 100, 'left': 0, 'width': 400, 'height': 300}
sct = mss()
while True:
sct_img = sct.grab(bounding_box)
cv2.imshow('screen', np.array(sct_img))
if (cv2.waitKey(1) & 0xFF) == ord('q'):
cv2.destroyAllWindows()
break
With this solution, I easily get 20+ frames/second.
For reference, check this link: OpenCV/Numpy example with mss