title: NSNotificationCenter - UIApplicationWillEnterForegroundNotification takes too long
body: <p>My code: </p>
  
  <pre><code>overide func viewDidLoad() {
     NSNotificationCenter.defaultCenter().addObserver(self, selector: "activeAgain", name: UIApplicationWillEnterForegroundNotification, object:nil)
  }
  
  func activeAgain() {
     println("ActiveAgain") //Comes immediately
     callSeparateFunction() //Takes 5s
  }
  </code></pre>
  
  <p>Why does the println comes immediately and the separate function call takes up to 5s? I have to call a new function when the app comes back to foreground. Thanks!</p>