Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux


import time

def t1():
    t1 = time.time()
    b = b''
    for _ in range(1_000_000):
        b += b'i'
    print(f'concat {time.time() - t1}')

def t2():
    t1 = time.time()
    b = bytearray()
    for _ in range(1_000_000):
        b += b'i'
    print(f'byte array concat {time.time() - t1}')


def t3():
    t1 = time.time()
    b = bytearray()
    for _ in range(1_000_000):
        b.extend(b'i')
    print(f'byte array extend {time.time() - t1}')


t1()
t2()
t3()



concat 36.097856521606445
byte array concat 0.04438519477844238
byte array extend 0.07430744171142578