This test is unfair! Concatenating just two strings is not a job for StringBuilder at all, and at higher numbers += plainly sucks.

Here are my results: (ms required to perform 1E+6 additions to a string)

Parts: 2
StringBuilder: 109.375
Concat: 109.375
Concat+Array: 140.625
+=: 93.75

Parts: 4
StringBuilder: 250
Concat: 203.125
Concat+Array: 265.625
+=: 250

Parts: 8
StringBuilder: 437.5
Concat: 359.375
Concat+Array: 484.375
+=: 656.25

Parts: 16
StringBuilder: 812.5
Concat: 656.25
Concat+Array: 937.5
+=: 1812.5

Parts: 32
StringBuilder: 1515.625
Concat: 1296.875
Concat+Array: 1843.75
+=: 5187.5

Parts: 64
StringBuilder: 2812.5
Concat: 2515.625
Concat+Array: 3453.125
+=: 16484.375

Parts: 128
StringBuilder: 5359.375
Concat: 4890.625
Concat+Array: 6703.125
+=: 57578.125

You see, already when concatenating 8 strings StringBuilder is better than += it clearly shows O(n) complexity while += shows O(n^2).

string.Concat itself seems wo be the fastest, but… We need to take into account that string.Concat requires an array of strings, and populating an array takes time too. This sole makes Concat + array operations about 10-20% slower than just StringBuilder.

So yeah, happy coding, just make sure your += is not inside a loop

Here is the code I ran to get the results: http://rafb.net/p/0hvZjC71.html

Regards,
Dmitry