Proof of Python's join is faster than string concatenation.

join_and_concat.py

>>> a = ["a","b","c"]
>>> def A():
...     return a[0]+a[1]+a[2]
... 
>>> def B():
...     return "".join(a)
... 
>>> import dis
>>> dis.dis(A)
  2           0 LOAD_GLOBAL              0 (a)
              3 LOAD_CONST               1 (0)
              6 BINARY_SUBSCR       
              7 LOAD_GLOBAL              0 (a)
             10 LOAD_CONST               2 (1)
             13 BINARY_SUBSCR       
             14 BINARY_ADD          
             15 LOAD_GLOBAL              0 (a)
             18 LOAD_CONST               3 (2)
             21 BINARY_SUBSCR       
             22 BINARY_ADD          
             23 RETURN_VALUE        
>>> dis.dis(B)
  2           0 LOAD_CONST               1 ('')
              3 LOAD_ATTR                0 (join)
              6 LOAD_GLOBAL              1 (a)
              9 CALL_FUNCTION            1
             12 RETURN_VALUE