At what point does using a StringBuilder become insignificant or an overhead?

Question

Recently I have found myself using StringBuilder for all string concatenations, big and small, however in a recent performance test I swapped out a colleague\'s stringOu

Answer 1

The rule that I follow is -

Use a StringBuilder when the number of concatenations is unknown at compile time.

So, in your case each StringBuilder was only appending a few times and then being discarded. That isn't really the same as something like

string s = String.Empty;
for (int i = 0; i < 10000; ++i)
{
    s += "A";
}

Where using a StringBuilder would drastically improve performance because you would otherwise be constantly allocating new memory.