Performance issue on iPhone 5

Hi guys, I'm getting some weird performance issues on iPhone 5 - the game lags when the user taps the screen to change direction. It doesn't happen on iPhone 6/iPod 6g/iPhone 4. Tried to experiment with disabling sounds, changing fixed timestep, but nothing helps. Here's my Update code which seems to be a problem:

     void Update ()
     {
 
         if (Input.GetButtonDown ("Fire1")) {
          if(started)
             {
                 gameController.IncScore ();
                 if (nextLos == 0) 
                 {
                     index += ballDir;
                     if (index == TrackPath.currentPath.angles.Length)
                         index = 0;
                     else if (index == -1)
                         index = TrackPath.currentPath.angles.Length - 1;
                     ballDir *= -1;
                     dir = Quaternion.Euler (0, 0, 180) * dir;
                 } 
                 else 
                 {
 
                     index += ballDir;
                     if (index == TrackPath.currentPath.angles.Length)
                         index = 0;
                     if (index == -1)
                         index = TrackPath.currentPath.angles.Length - 1;
 
                        dir = Quaternion.Euler(0, 0, TrackPath.currentPath.angles[index] * ballDir) * dir;
                     pathNo += ballDir;
                     if (pathNo == -1)
                         pathNo = TrackPath.currentPath.angles.Length - 1;
                     else if (pathNo == TrackPath.currentPath.angles.Length)
                         pathNo = 0;
                 }
             
             }
             else {
                 started=true;
                 gameController.tapToStart.GetComponent<PopUpEffect> ().StartCoroutine ("SimplePopDown");
                 if (nextLos == 0) {
                     ballDir = -1;
                     index = 1;
                     dir = TrackPath.currentPath.startDir2;
                 } else {
                     ballDir = 1;
                     dir = TrackPath.currentPath.startDir1;
                 }            
             }
             if (pathNo == 2)
                 tpJumpsCount++;
             else {
                 keyCountdown--;
                 if (keyCountdown == 0) {
                     gameController.SpawnKey ();
                     keyCountdown = keyDelay;
                 }
             }
             nextLos = next2Los;
             next2Los = Random.Range (0, 4);
             if (v < 7f)
                 v += .2f;
             player.velocity = dir*v;
         }
     }

void IncScore() is just score++; ScoreView.text=score+""; What's interesting, the longer the game is running, the smoother it gets - after couple minutes running it's perfect. It also seems to run a little better on iPhone 5 with ios 9 - lags on tap are not that often - but still happens. On ihone 5 with ios 8 - the lag happens almost each time you hit a screen.

Can you see anything suspicious? Or you had similar problem with iPhone 5?