You can do it on Global.asax, look at the article
void Application_BeginRequest(object sender, EventArgs e)
{
HttpContext.Current.Items["renderStartTime"] = DateTime.Now;
}
void Application_EndRequest(object sender, EventArgs e)
{
DateTime start = (DateTime) HttpContext.Current.Items["renderStartTime"];
TimeSpan renderTime = DateTime.Now - start;
HttpContext.Current.Response.Write("<!-- Render Time: " + renderTime + " -->");
}