Calling Instantiate() freezes the game for 3-5 seconds after build, but not in the editor

Calling the following function for the first time the game freezes for 5 seconds. After the freeze I can call it multiple times and it works fine, until I restart the game, then the problem occurs again. In the editor there is no problem at all.

 void LoadLevelSelect() {
         int HowManyMaps = mapCount;
 
         //currently the value of mapCount is 5 
         //instantiateing blocks in 5 rows and 7 columns

         for (int i = 4; i >= -4; i = i-2) {
             for (int j = -6; j <= 6; j = j+2) {
 
                 if (HowManyMaps != 0) {
                     Vector3 Where = new Vector3 (j, 0, i);
                     Instantiate (MapSelect, Where, MapSelect.transform.rotation);
                     HowManyMaps --;
                 } else
                     break;
             }
         }
 }

Any idea how to solve this little issue? Sorry if there is an answered question for this problem, but i did not find anything useful for this... Thanks in advance