gistfile1.cs

Stopwatch sw = new Stopwatch();
string str = new string(' ', 1024 * 1024 * 8);

sw.Restart();
{
    var ret = str
        .Select(c => string.Format("[{0}]", c))
    ;
    string.Join("", ret);
}
Console.WriteLine("{0} milliseconds", sw.ElapsedMilliseconds);
// 2476 milliseconds

sw.Restart();
{
    var ret = str
        .Aggregate(new StringBuilder(), (sb, c) => sb.AppendFormat("[{0}]", c))
        .ToString()
    ;
}
Console.WriteLine("{0} milliseconds", sw.ElapsedMilliseconds);
// 1343 milliseconds

Console.ReadKey();