Why does my animation jump/lag when activating a game object?

I am trying to move a game object and its children off screen while simultaneously moving a newly initialized object onto the screen using this co-routine:

 public IEnumerator FromPacksToLevels (){
         GameController.Screens.LevelSelect.SetActive (true);
         IEnumerator TOUT = GameController.Screens.PackSelect.GetComponent<LoadPacks> ().TransitionR2L ();
         IEnumerator TIN = GameController.Screens.LevelSelect.GetComponent<LoadLevels> ().TransitionR2L ();
         StartCoroutine (TOUT);
         StartCoroutine (TIN);
         yield return new WaitForSeconds(0.55f);
         GameController.Screens.PackSelect.SetActive (false);
     }

The transitions (TransitionR2L) look like this:

     public IEnumerator TransitionR2L(){
         IEnumerator Slide = BasicAnimations.RelativeMove (gameObject, slideLeft, .5f);
         StartCoroutine (Slide);
         yield return null;
     }

This works ok, but there is a noticeable lag/jump in the animation the first time that it runs. Successive transitions are smooth (if I go back and forth between levels and packs) but the first one is not.

Is there something I need to do or check to see if an object and its children are fully initialized before initiating the transition animation?