Почему нельзя использовать свободный язык наstring?

Например:

var x = "asdf1234";
var y = new string(x.TakeWhile(char.IsLetter).ToArray());

Нет ли лучшего способа конвертироватьIEnumerable<char> вstring?

Вот тест, который я сделал:

class Program
{
  static string input = "asdf1234";
  static void Main()
  {
    Console.WriteLine("1000 times:");
    RunTest(1000, input);
    Console.WriteLine("10000 times:");
    RunTest(10000,input);
    Console.WriteLine("100000 times:");
    RunTest(100000, input);
    Console.WriteLine("100000 times:");
    RunTest(100000, "ffff57467");


    Console.ReadKey();

  }

  static void RunTest( int times, string input)
  {

    Stopwatch sw = new Stopwatch();

    sw.Start();
    for (int i = 0; i < times; i++)
    {
      string output = new string(input.TakeWhile(char.IsLetter).ToArray());
    }
    sw.Stop();
    var first = sw.ElapsedTicks;

    sw.Restart();
    for (int i = 0; i < times; i++)
    {
      string output = Regex.Match(input, @"^[A-Z]+", 
        RegexOptions.IgnoreCase).Value;
    }
    sw.Stop();
    var second = sw.ElapsedTicks;

    var regex = new Regex(@"^[A-Z]+", 
      RegexOptions.IgnoreCase);
    sw.Restart();
    for (int i = 0; i < times; i++)
    {
      var output = regex.Match(input).Value;
    }
    sw.Stop();
    var third = sw.ElapsedTicks;

    double percent = (first + second + third) / 100;
    double p1 = ( first / percent)/  100;
    double p2 = (second / percent )/100;
    double p3 = (third / percent  )/100;


    Console.WriteLine("TakeWhile took {0} ({1:P2}).,", first, p1);
    Console.WriteLine("Regex took {0}, ({1:P2})." , second,p2);
    Console.WriteLine("Preinstantiated Regex took {0}, ({1:P2}).", third,p3);
    Console.WriteLine();
  }
}

Результат:

1000 times:
TakeWhile took 11217 (62.32%).,
Regex took 5044, (28.02%).
Preinstantiated Regex took 1741, (9.67%).

10000 times:
TakeWhile took 9210 (14.78%).,
Regex took 32461, (52.10%).
Preinstantiated Regex took 20669, (33.18%).

100000 times:
TakeWhile took 74945 (13.10%).,
Regex took 324520, (56.70%).
Preinstantiated Regex took 172913, (30.21%).

100000 times:
TakeWhile took 74511 (13.77%).,
Regex took 297760, (55.03%).
Preinstantiated Regex took 168911, (31.22%).

Вывод: я сомневаюсь, что лучше предпочесть, я думаю, что я пойду наTakeWhile который является самым медленным только при первом запуске.

Во всяком случае, мой вопрос, есть ли способ оптимизировать производительность, ограничивая результатTakeWhile функция.