StringTimes.cs

namespace StringTimes
{
    using System;

    public class Program
    {
        private static void Main(string[] args)
        {
            const int Iterations = 10000000;

            string Icon = "<i class=\"fa fa-bolt red\"></i>";
            string Risk = "Critical";

            Time(Iterations, "string.Format", () =>
            {
                string s = string.Format("{0} {1}", Icon, Risk);
            });


            Time(Iterations, "concatenation", () =>
            {
                string s = Icon + " " + Risk;
            });

            Time(Iterations, "StringBuilder", () =>
            {
                var sb = new System.Text.StringBuilder();
                sb.Append(Icon);
                sb.Append(" ");
                sb.Append(Risk);
                string s = sb.ToString();
            });

            Console.Write("Press any key to continue . . .");
            Console.ReadKey(true);
        }

        private static void Time(int iterations, string method, Action action)
        {
            System.Console.WriteLine("Timing {0} iterations of {1}", iterations, method);

            var sw = System.Diagnostics.Stopwatch.StartNew();

            for (int i = 0; i < iterations; i++)
            {
                action.Invoke();
            }

            GC.Collect();
            GC.Collect();

            var elapsed = sw.Elapsed;

            Console.WriteLine("{0} took {1}", method, elapsed);
            Console.WriteLine();
        }
    }
}

/*
Timing 10000000 iterations of string.Format
string.Format took 00:00:05.5955579

Timing 10000000 iterations of concatenation
concatenation took 00:00:01.5413132

Timing 10000000 iterations of StringBuilder
StringBuilder took 00:00:06.7475308
*/