C#

Func<string, object> convert = str =>
            {
                int value = 0;
                return int.TryParse(str, out value) ?  (object)value : str;
            };

  Func<string, object> convert = str =>
  {
      try { return int.Parse(str); }
      catch { return str; }
 };//My function (one borrowed from link above)

Func<string, object> convert = str =>
{
    int value = 0;
    return int.TryParse(str, out value) ? (object)value : str;
};//Emardini's function that works 'properly', i.e fast

txtOutput.Lines = sorted;

string[] data = File.ReadAllLines(@"D:\Temp\MyLargeTextFile.txt");
Stopwatch s1 = new Stopwatch();
Stopwatch s2 = new Stopwatch();
s1.Start();
textBox1.Lines = data;
s1.Stop();
s2.Start();
StringBuilder sb = new StringBuilder();
foreach (string s in data)
    {
    sb.Append(s);
    sb.Append("\r\n");
    }
textBox2.Text = sb.ToString();
s2.Stop();
Console.WriteLine("{0} lines: {1}/{2}", data.Length,s1.Elapsed, s2.Elapsed);

22777 lines: 00:00:01.5580246/00:00:01.5555734

x: 
dt←$DateTime:Now;
 sb←#new $StringBuilder ⍬;
 {sb:AppendLine (⍵:ToString)}¨⍳x;
 ($DateTime:Now)-dt 

builder.AppendFormat("{0}\r\n", s);

List<string> sl = new List<string> { "wo", "ab", "c"};

sl.Sort();

textBox1.Lines = sl.ToArray();