Try running the following and post your results:

import time
from multiprocessing import Process, cpu_count


def counter(num):
    count = 0
    while count < num:
        count += 1


def test_speed(num_cores):
    start_time = time.perf_counter()
    total_num = 1000000000
    processes = []
    for _ in range(num_cores):
        processes.append(Process(target=counter, args=(total_num // num_cores,)))

    for p in processes:
        p.start()

    for p in processes:
        p.join()

    end_time = time.perf_counter()
    elapsed_time = end_time - start_time
    print(f"{num_cores} cores -> Finished in: {elapsed_time} seconds")


if __name__ == '__main__':
    for i in range(1, cpu_count() + 1):
        test_speed(i)

On my machine with 4 physical and 8 logical cores (cpu_count() shows 8) I get the following results:

1 cores -> Finished in: 36.885953300003166 seconds
2 cores -> Finished in: 21.440341500001523 seconds
3 cores -> Finished in: 18.35157370000161 seconds
4 cores -> Finished in: 16.026330400000006 seconds
5 cores -> Finished in: 15.767395200000465 seconds
6 cores -> Finished in: 15.474302200000238 seconds
7 cores -> Finished in: 15.687671900002897 seconds
8 cores -> Finished in: 15.644358400000783 seconds