Async controller doesn't work in upgraded project from MVC 3 to MVC 5

Question

I upgraded all stuff using Upgrading a project to MVC 5 but method

public async Task<ActionResult> Index()
{
    await Task.Delay(500);
    [...]
    return View(model);
}

Gives me results like:

System.Threading.Tasks.Task`1[System.Web.Mvc.ActionResult]

instead of normal view.