[C#] the performance of String & StringBuilder

This is just a note for myself about the performance of String and StringBuilder in C#.
I always know that StringBuilder is recommended when adding strings dynamically,
but how huge the difference is?

Following is the sample code.

            DateTime start = DateTime.Now;
            string str = "1234567890";
            string longer = "";
            for (int i = 0; i < 100000; i++)
            {
                longer += str;
            }

            DateTime end = DateTime.Now;
            TimeSpan ts = new TimeSpan(end.Ticks - start.Ticks);
            Console.WriteLine(ts);

            DateTime start2 = DateTime.Now;
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 100000; i++)
            {
                sb.Append(str);
            }
            DateTime end2 = DateTime.Now;
            TimeSpan ts2 = new TimeSpan(end2.Ticks - start2.Ticks);
            Console.WriteLine(ts2);
            Console.ReadKey();

And this is the result.