Random string generating function
np.random.choice takes an additional argument specifying how many elements should be chosen. You can also use the built-in module string to save having to spell out the alphabet:

import numpy as np
import string

ALPHABET = np.array(list(string.ascii_lowercase + ' '))


def generate_guess(sentence):
    return np.random.choice(ALPHABET, size=len(sentence))


I also made the alphabet a constant to avoid having to build it again and again.

Your code was not faster because you did not take advantage of the vector-wise operations of numpy. Neither your starting array, the alphabet, nor your output array were actual numpy arrays.