How to increase rate of a falling object in pooling object?

i m currently working on a script where i want to drop random objects from a list of prefabs. i am using pool objects for this. The problem is as follow: i want these objects one by one when game starts. The rate of falling objects should be slow when game starts. But as time passes, the objects shold fall with increasing more speed. How to do that?

using UnityEngine; using System.Collections; using System.Collections.Generic; using System.Linq;

public class Poolingscript2 : MonoBehaviour {

 public    GameObject[] allprefabs;   //array of prefab
 public    List<GameObject> bullets ; 
 public    int listsize;
 GameObject go;
 // Use this for initialization
 void Start ()
 {
     createlist ();

 }

 void createlist() //creates list of gameobjects from array of prefabs
 {
      allprefabs = Resources.LoadAll<GameObject> ("Prefab"); //reload prefabs in array

     for (int i=0; i<listsize; i++) 
     {
         foreach (GameObject allprifab in allprefabs) 
         {
             go = Instantiate(allprifab)as GameObject; //make object from aray
             go.SetActive(false);
             bullets.Add (go); //add to list

         }

     }

 }


 GameObject temp;
 void fallobject()
 {
     int randm = Random.Range(0,listsize); //select random number
     temp = bullets.ElementAt(randm);      // assign number and select random object
     bullets.Remove(temp);   // remove that random object
     temp.transform.position = transform.position; 
     temp.SetActive(true);   //activate, so it falls down
     Debug.Log ("removed element no: " +randm);    

 }
 public void Update()
 {
         
 }

 public void addtolist(GameObject g)  //function for adding fallen object back to list
 {
     g.SetActive (true);
     bullets.Add (g);

 }

}