代码验证C#执行”文件打开关闭操作“耗时

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;
using System.Threading;

namespace ConsoleApplication1
{
    public class Program
    {
        static void Main(string[] args)
        {
            int index = 0;
            long i = 0;
            var stopWatch = new Stopwatch();
            //StreamWriter f = new StreamWriter(@"D:\sum.txt", false);
            for (i = 0; i < 100000; i++)
            {
                stopWatch.Start();
                if (i == 10000)
                {
                    stopWatch.Stop();
                    Console.WriteLine("Program run" + stopWatch.ElapsedMilliseconds + "ms.");
                    Thread.Sleep(10000);
                }
                i += 1;
                Console.WriteLine(i); 
                StreamWriter f = new StreamWriter(@"D:\sum.txt", false);
                index += 1;
                f.Write(index);
                f.Close();
            }
            //f.Close();
            Console.ReadKey();
        }
    }
}