Tempo de execução de uma Action/ASP.NET MVC 'manualmente'

gistfile1.cs

//TesteApp\TimeExecutionAttribute.cs
using System.Diagnostics;

namespace TesteApp
{
    public class ExecutionTimeAttribute : ActionFilterAttribute
    {
        private Stopwatch _stopwatch;

        public ExecutionTimeAttribute()
        {
            _stopwatch = new Stopwatch();
        }

        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            _stopwatch.Start();
        }

        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            _stopwatch.Stop();

            filterContext.Controller.ViewData["ExecutionTime"] = _stopwatch.Elapsed.ToString();
        }
    }
}

//TesteApp\Controllers\HomeController.cs
namespace TesteApp.Controllers
{
    [ExecutionTime]
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            //processamento
            return View();
        }
    }
}

//TesteApp\Views\Home\Index.aspx
<h2>Teste</h2>
<p>
    Tempo gasto no processamento <%:ViewData["ExecutionTime"]%>...
</p>