Different way to spawn prefabs

hello everyone! actually i have a working code to spawn my prefabs object on the 2D fields, but looks using too much memory, when i install it on iphone i'm getting a low frame rate on the first second of the game, so should be this part of code, someone know a different way to do the same things?

sorry for my bad English.

     public int CoinPoolSize = 20;
     public float spawnRate = 2f;
     public GameObject coinPrefabs;
     public float coinMin = -6f;
     public float coinMax = 10f;
 
 
     private GameObject[] coin;
     private Vector2 coinPoolPosition = new Vector2(-15f,-25f);
     private float timeSinceLastSpawned;
     private float spawnXposition = 10f;
     private int currentCoin = 0;
 
 
 
 
     // Use this for initialization
     void Start () 
     {
         coin = new GameObject[CoinPoolSize];
         for(int i = 0; i < CoinPoolSize; i++)
         {
         coin [i] = (GameObject)Instantiate (coinPrefabs, coinPoolPosition, Quaternion.identity);
         }
     }
     
     // Update is called once per frame
     void Update () 
     {
         timeSinceLastSpawned += Time.deltaTime;
         if(GameControl.instance.gameOver == false && timeSinceLastSpawned >= spawnRate) 
         {
             timeSinceLastSpawned = 0;
             float spawnYposition = Random.Range(coinMin,coinMax);
             coin [currentCoin].transform.position = new Vector2(spawnXposition, spawnYposition);
             currentCoin ++;
             if(currentCoin >= 20) 
             {
                 currentCoin = 0;
 
             }
         }
     }