gistfile1.cs

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {

            var lista = new List<string>();

            for (int i = 0; i < 700000; i++)
            {
                lista.Add(RandomString(100));
            }

            Stopwatch stopWatch = new Stopwatch();
            stopWatch.Start();

            var achados = lista.Where(x => x.Contains("A")).Take(10);

            stopWatch.Stop();
            Console.WriteLine(stopWatch.ElapsedMilliseconds);
            Console.Read();
        }

        private static Random random = new Random((int)DateTime.Now.Ticks);//thanks to McAden
        private static string RandomString(int size)
        {
            StringBuilder builder = new StringBuilder();
            char ch;
            for (int i = 0; i < size; i++)
            {
                ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
                builder.Append(ch);
            }

            return builder.ToString();
        }
    }
}