This measures the time for conversions of integers to strings and logging them in the console.

IntegerConversionTest.cs

using System;
using System.Diagnostics;

namespace WriteLineSpeed
{
    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch stopwatch = new Stopwatch();

            Console.WriteLine("Conversion tests");
            Console.WriteLine("i + \"\"");
            stopwatch.Start();
            for (int i = 0; i < 1000000; i++)
            {
                string s = i + "";
            }
            Console.WriteLine(stopwatch.ElapsedMilliseconds);

            Console.WriteLine("i.ToString()");
            stopwatch.Restart();

            for (int i = 0; i < 1000000; i++)
            {
                string s = i.ToString();
            }
            Console.WriteLine(stopwatch.ElapsedMilliseconds);

            Console.WriteLine("Console.WriteLine tests");
            Console.WriteLine("i+\"\"");
            stopwatch.Restart();
            for (int i = 0; i < 1000; i++)
            {
                Console.WriteLine(i + "");
            }
            Console.WriteLine(stopwatch.ElapsedMilliseconds);
            Console.WriteLine("---------------------------------");

            Console.WriteLine("i");
            stopwatch.Restart();
            for (int i = 0; i < 1000; i++)
            {
                Console.WriteLine(i);
            }

            Console.WriteLine(stopwatch.ElapsedMilliseconds);
            Console.ReadKey();
        }
    }
}