ASP.NET MVC 5 concurrent requests are queued even with disabled Session

Question

Before thinking about downvoting or telling me "google it", please read the problem more carefully. This is old/classic problem but old/classic solution is no longer working. Here is very simple scenario to reproduce in Visual Studio 2013/2015:

1) Create ASP.NET Web application using MVC template:

2) Open Controllers\HomeController.cs and add attribute to controller and "Sleep" action:

[SessionState( System.Web.SessionState.SessionStateBehavior.Disabled)]
public class HomeController : Controller
{
    public ActionResult Sleep(int? time)
    {
        System.Threading.Thread.Sleep(time ?? 3000);
        return Content("OK");
    }

    public ActionResult Index()
    {
...

3) Open file: Views\Home\Index.cshtml and add/replace content html with the following :

4) Run it (does not matter if you're using IIS or IIS Express or Vs Dev Server) - Open Home/Index. Click F12 to open dev tool, open network tab. On the Home page click "Request" button twice fast. You can see that second request takes almost 6 seconds:

In Debug mode in controller you can see that Session is null:

Cookies are totally empty (ASP.NET Session Id is absent)

Please let me know what I'm missing?

Adding the setting below to web.config does not help either:

<sessionState mode="Off"/>
<pages enableSessionState="ReadOnly"/>

Answer 1

Concurrent parallel requests worked for me when I decorated my controller with this attribute

[SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]

It works better than the above disabling session state and was added back in MVC 3. More info here