isalpha_performance.py

import string
import re
import timeit

def remove_unwanted_symbols_1(text):
    test_list = []
    for symbol in text.lower():
        if symbol in string.ascii_letters:
            test_list.append(symbol)
    return ''.join(test_list)

def remove_unwanted_symbols_2(text):
    test_list = []
    for symbol in text.lower():
        if symbol == "-" or symbol == "`":
            continue
        test_list.append(symbol)
    return ''.join(test_list)

def remove_unwanted_symbols_3(text):
    test_list = []
    for symbol in text.lower():
        if symbol.isalpha():
            continue
        test_list.append(symbol)
    return ''.join(test_list)

if __name__ == '__main__':
    with open('lorem.txt') as f:
        text = f.read()

    print('Letters count is', len(text))

    print(timeit.Timer(lambda: remove_unwanted_symbols_1(text)).timeit(number=5))
    print(timeit.Timer(lambda: remove_unwanted_symbols_2(text)).timeit(number=5))
    print(timeit.Timer(lambda: remove_unwanted_symbols_3(text)).timeit(number=5))

Результат вычислений на моей машине:

$ python3 test.py 
Letters count is 28367
0.03142735400251695
0.030193180002243025
0.02767529800257762

Вывод: дело не в isalpha