See https://www.reddit.com/r/Python/comments/84vs8x/why/

test_threading.py

import threading
from time import perf_counter as time
import random
from statistics import median, stdev

results = {'thread': [], 'call': []}

end = 0
def inside():
    global end
    for n in range(1000000):
        pass
    end = time()

for _ in range(1000):
    r = list(results.items())
    random.shuffle(r)
    for k, result_list in r:
        if k == 'thread':
            t_inside = threading.Thread(target=inside)
            start = time()
            t_inside.start()
            t_inside.join()
        elif k == 'call':
            start = time()
            inside()
        result_list.append(end - start)

def fmt_time(t):
    return format(t * 1000, 'f')

print('(all results in µs)')
print('-' * 6, ' min       max       median    stdev')
for k, result in sorted(results.items(), key=lambda pair: min(pair[1])):
    print(f'{k:7}', fmt_time(min(result)), fmt_time(max(result)), fmt_time(median(result)), fmt_time(stdev(result)))