multiprocessing and time.perf_counter

I'm trying to learn multiproccessing in python and I'd like to see how long my program takes to run the code by using multiproccessing, but I can't understand why the time.perf_counter function prints two really small numbers (6.00004568696022e-07 and 1.200009137392044e-06) and after that the actual amount of seconds (18.546351400000276) of the duration of the program. Can you expalin me why?

Thanks

import time
from multiprocessing import Process

start = time.perf_counter()

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

if __name__ == '__main__':
    a = Process(target = counter, args = (500000000,))
    b = Process(target = counter, args = (500000000,))
    a.start()
    b.start()
    a.join()
    b.join()

print(time.perf_counter() - start)