runtime: HttpWebRequest hits 1s timeout when resolving http://localhost on Win10 (.NET Framework is fast)

Consider the following method (error/exception handling omitted for brevity:

public virtual string MakeHttpRequest(Uri fullUri, string httpMethod, string requestBody)
{
    string responseString = string.Empty;
    HttpWebRequest request = HttpWebRequest.Create(fullUri) as HttpWebRequest;
    request.Method = httpMethod;
    request.Timeout = 30000;
    request.Accept = "application/json,image/png";
    if (request.Method == "POST")
    {
        string payload = requestBody;
        byte[] data = Encoding.UTF8.GetBytes(payload);
        request.ContentType = "application/json;charset=utf-8";
        Stream requestStream = request.GetRequestStream();
        requestStream.Write(data, 0, data.Length);
        requestStream.Close();
    }

    HttpWebResponse webResponse = request.GetResponse() as HttpWebResponse;
    using (Stream responseStream = webResponse.GetResponseStream())
    using (StreamReader responseStreamReader = new StreamReader(responseStream, Encoding.UTF8))
    {
        responseString = responseStreamReader.ReadToEnd();
    }
    return responseString;
}

Executing this method against a server running on localhost yields wildly different execution times when run via the full .NET Framework vs. .NET Core. I have a demo project that shows the discrepancy, making calls to a locally running HTTP server, that server being the Chromium project’s chromedriver.exe, used to automate the Chrome browser. When running the executables created by that project, you can see the following typical outputs (as run on Windows 10, build 16288):

Run against .NET Core 2.0:

Starting test… Starting ChromeDriver 2.32.498550 (9dec58e66c31bcc53a9ce3c7226f0c1c5810906a) on port 9515 Only local connections are allowed. Started session 946394531499259b7442073c0e26060d Navigating to http://www.google.com Navigation complete Making 10 HTTP calls to localhost, logging the elapsed time… Elapsed time for HTTP call: 1038 milliseconds Elapsed time for HTTP call: 1016 milliseconds Elapsed time for HTTP call: 1015 milliseconds Elapsed time for HTTP call: 1011 milliseconds Elapsed time for HTTP call: 1012 milliseconds Elapsed time for HTTP call: 1024 milliseconds Elapsed time for HTTP call: 1013 milliseconds Elapsed time for HTTP call: 1027 milliseconds Elapsed time for HTTP call: 1013 milliseconds Elapsed time for HTTP call: 1014 milliseconds Test finished. Press <Enter> to exit.

Running against the full .NET Framework 4.5:

Starting test… Starting ChromeDriver 2.32.498550 (9dec58e66c31bcc53a9ce3c7226f0c1c5810906a) on port 9515 Only local connections are allowed. Started session 81c2bbd21a0d89354b2dead8d52ee982 Navigating to http://www.google.com Navigation complete Making 10 HTTP calls to localhost, logging the elapsed time… Elapsed time for HTTP call: 35 milliseconds Elapsed time for HTTP call: 7 milliseconds Elapsed time for HTTP call: 12 milliseconds Elapsed time for HTTP call: 5 milliseconds Elapsed time for HTTP call: 4 milliseconds Elapsed time for HTTP call: 7 milliseconds Elapsed time for HTTP call: 4 milliseconds Elapsed time for HTTP call: 6 milliseconds Elapsed time for HTTP call: 5 milliseconds Elapsed time for HTTP call: 5 milliseconds Test finished. Press <Enter> to exit.

I would expect similar performance between framework versions. Absent that, I would expect there to be settings to modify to the HttpWebRequest object that would yield similar performance as a workaround.

[EDIT] C# syntax highlighing by @karelz