Performance drop in multiple async stream writing of http post request

Question

Recently I am testing http "requests per second" difference on Threading and Async/Await.

Using Async/Await is always slighty better(~30%) than Threading (2000-3000 requests per second) until I change the httpwebrequest from "GET" to "POST"

Threading version is still normal (~2000-3000 requests per seconds) with high thread amount like 2000 threads

Async/Await version becomes slow and even stuck under high amount of async task situation( ~500 requests per second, some tasks (~1%) do not finish (unreasonable time like 1 minute) although all other tasks were all finished very early)

Please shed some light on this, I can barely think of a reason why this happens.

Here's the function that stuck

using (Stream postStream = await req.GetRequestStreamAsync())
{
    await postStream.WriteAsync(bytes, 0, bytes.Length); //This is the function that stuck in async version
}

Here's my test code

public static Amount = 0;
public void StartSync(int threadAmount)
{
            Amount = 0;
            Thread[] thread = new Thread[threadAmount];
            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < thread.Length; i++)
            {
                thread[i] = new Thread(() =>
                {
                    SyncTest();
                });
                thread[i].IsBackground = true;
                thread[i].Start();
            }
            for (int j = 0; j < thread.Length; j++)
            {
                thread[j].Join();
            }
            sw.Stop();
            MessageBox.Show("Total requests: " + amount + "\r\nTotal seconds: " + (sw.ElapsedMilliseconds / 1000.0) + "\r\nRequests per second: " + Convert.ToDouble(amount) / (sw.ElapsedMilliseconds / 1000.0));
}
public void StartAsync(int threadAmount)
{
            Amount = 0;
            Task[] task = new Task[threadAmount];
            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < threadAmount; i++)
            {
                task[i] = AsyncTest();
            }

            await Task.WhenAll(task);
            sw.Stop();
            MessageBox.Show("Total requests: " + amount + "\r\nTotal seconds: " + (sw.ElapsedMilliseconds / 1000.0) + "\r\nRequests per second: " + Convert.ToDouble(amount) / (sw.ElapsedMilliseconds / 1000.0));
}
public async Task<bool> AsyncTest()
{

            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("http://www.google-analytics.com/__utm.gif");
            req.Method = "POST";
            req.AllowAutoRedirect = false;
            byte[] bytes = Encoding.ASCII.GetBytes("test");
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentLength = bytes.Length;


            using (Stream postStream = await req.GetRequestStreamAsync())
            {
                await postStream.WriteAsync(bytes, 0, bytes.Length); //This is the function to be investigated. Adding this line slows down async side significantly
            }

            using (HttpWebResponse res = (HttpWebResponse)await req.GetResponseAsync())
            using (Stream responseStream = res.GetResponseStream())
            {
                StreamReader sr = new StreamReader(responseStream, Encoding.ASCII);
                string resp = await sr.ReadToEndAsync();
            }
            Amount++;
            return true;
}

public bool SyncTest()
{
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("http://www.google-analytics.com/__utm.gif");
            req.Method = "POST";
            req.AllowAutoRedirect = false;
            byte[] bytes = Encoding.ASCII.GetBytes("test");
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentLength = bytes.Length;
            using (Stream oStreamOut = req.GetRequestStream())
            {
                oStreamOut.Write(bytes, 0, bytes.Length);
            }
            using (HttpWebResponse webResponse = (HttpWebResponse)req.GetResponse())
            {
                using (Stream responseStream = webResponse.GetResponseStream())
                {
                    StreamReader sr = new StreamReader(responseStream, Encoding.ASCII);
                    string resp = sr.ReadToEnd();
                }
            }
            Amount++;
            return true;
}

Update 1:

I tested WriteAsync in memorystream with high amount of async tasks but the slow down did not happen

public async Task<bool> AsyncWrite()
{
    byte[] bytes = Encoding.ASCII.GetBytes("test");
    using (MemoryStream ms = new MemoryStream())
    {
        await ms.WriteAsync(bytes, 0, bytes.Length);
    }
}

Update 2:

I plugged out my router, async version becomes normal, no more stuck. Only with router connected, the weird slow down happens. I cannot explain why this happened to async version only but not threading...