asp.net web/app.config read/write performance

    public class HomeController : Controller
    {
        const int IterationCount = 1000;
        private Stopwatch stopwatch = new Stopwatch();
        public ActionResult Index()
        {
            if(Session["mykey"] == null)
                Session["mykey"] = Guid.NewGuid();

            ViewBag.FirstWrite = TestWritePerformance();
            ViewBag.FirstReadAfterWrite = TestReadPerformance();

            TestWriteAverage();
            TestReadAverage();

            return View();
        }

        private void TestReadAverage()
        {
            long readSum = 0;
            for (int i = 0; i < IterationCount; i++)
            {
                readSum += TestReadPerformance();
            }
            ViewBag.AverageRead = readSum/IterationCount;
        }

        private void TestWriteAverage()
        {
            long writeSum = 0;
            for (int i = 0; i < IterationCount; i++)
            {
                writeSum += TestWritePerformance();
            }
            ViewBag.AverageWrite = writeSum/IterationCount;
        }

        private long TestReadPerformance()
        {
            stopwatch.Restart();
            var configFile = WebConfigurationManager.OpenWebConfiguration("~");

            var readAfterWrite = configFile.AppSettings.Settings["MYKEY"].Value;
            stopwatch.Stop();

            return stopwatch.ElapsedMilliseconds;
        }

        private long TestWritePerformance()
        {
            var guid = Guid.NewGuid().ToString();

            var configFile = WebConfigurationManager.OpenWebConfiguration("~");
            var settings = configFile.AppSettings.Settings;

            stopwatch.Restart();
            settings["MYKEY"].Value = guid;
            configFile.Save(ConfigurationSaveMode.Minimal);
            ConfigurationManager.RefreshSection("appSettings");
            stopwatch.Stop();

            return stopwatch.ElapsedMilliseconds;
        }
}