How to get a smooth movement of GameObjects

Hi, All. I created simple 2D project. In the editor it works fine. But when I run it on devices (Android, iOS), it slows down periodically. Movement is not smooth, objects jerk. In android FPS is 60, but periodically it drops to 40 and game freezing. Unity 5.3.3f1 Personal. Project is very simple: 1 prefab with Rigidbody2d and 3 scripts, but I can't understand the problem. Can somebody help?

CreateGame.cs:

 using System;
 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 
 public class CreateGame : MonoBehaviour {
 
     public GameObject cube;
 
     private List<GameObject> listCubes;
     private int CUBES_COUNT = 40;
 
     // Use this for initialization
     void Start () {
 
         //Create Object Pool
         createPool ();
 
         //Get Cube from Pool
         Invoke ("CreateCube", 0f);
     }
 
     //Create Pool
     void createPool(){
         for (int i = 0; i < CUBES_COUNT; i++) {
             ObjPool.instance.cubes.putObj((GameObject)Instantiate(cube));
         }
     }
 
     void CreateCube()
     {
         //get Cube from pool
         ObjPool.instance.cubes.getObj ();
         //Invoke itself
         Invoke("CreateCube", UnityEngine.Random.Range (0.2f, 0.5f));
     }
         
     // Update is called once per frame
     void Update () {
         //get Android back button
         if (Application.platform == RuntimePlatform.Android) {if (Input.GetKey(KeyCode.Escape)) { Application.Quit(); }}
     }
 }
 

ObjectPool.cs:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System;
 
 
 public class ObjPool : MonoBehaviour {
 
     public static ObjPool instance { get; private set; }
 
 
     public class Pool {
         private List<GameObject> pool;
         public Pool(){
             pool = new List<GameObject>();
         }
 
         public void putObj (GameObject obj){
             obj.SetActive (false);
             pool.Add (obj);
         }
             
         public GameObject getObj(){
             if (pool.Count > 0)
             {
                 GameObject pooledObject = pool[0];
                 pool.RemoveAt(0);
                 pooledObject.transform.parent = null;
                 pooledObject.SetActive(true);
 
                 return pooledObject;
             }
             return null;
         }
     }
 
     public Pool cubes;
 
     void OnEnable()
     {
         instance = this;
         cubes = new Pool ();
     }
 
 
 }

MoveCube.cs: using UnityEngine; using System.Collections;

 public class MoveCube : MonoBehaviour {
 
 
     private float range = 4f;
 
     private Vector2 velocity;
 
     private Vector3 initPosition;
 
     private Rigidbody2D rigidBody2d;
 
 
     void Start () {
         rigidBody2d = GetComponent<Rigidbody2D> ();
 
     }
 
 
     void OnEnable(){
         Init ();
 
     }
 
     public void Init()
     {
         if (initPosition == Vector3.zero) {
             initPosition = transform.position;
         }
         transform.position = new Vector3(initPosition.x, initPosition.y - range * Random.value, initPosition.z);
         transform.eulerAngles = Vector3.zero;
         transform.localScale = new Vector3(1, 1, 1);
 
 
     }
 
     void FixedUpdate()
     {
         velocity = new Vector2(-10, 0);
         rigidBody2d.velocity = velocity;
 
     }
         
 
     // Update is called once per frame
     void Update () {
         // Die by being out off screen
         if (transform.position.x < -13)
         {
             ObjPool.instance.cubes.putObj(gameObject);
         }
     }
 
 
 
 }