Resources.Load have a bad performance

I'm trying to play a movie in Android by using SpriteRenderer because MovieTexture can't be used for Android. Basically, I convert the movie into images and load it as Sprite and assign it to a SpriteRenderer every frame. I put the images in Resources/Movie folder and use this code to load the Image. (Variable counter is what frame the movie currently playing)

 Sprite GetFrame(){
     Sprite newSprite = Resources.Load("Movie/picture" + counter.ToString("0000"), typeof(Sprite)) as Sprite;
     return newSprite;
 }

At the beginning, the movie play smoothly, but after 10 seconds or so the movie starting to lag. I try to unload the resource every 2 seconds using Resources.UnloadUnusedAssets(); but the problem still exist. Here is my full code for this.

 using UnityEngine;
 using System.Collections;
 
 public class CPlayButton : MonoBehaviour {
     private const int FPS = 24;
     private const int UNLOAD_TIME = 2;
 
     public SpriteRenderer movieTexture;
     
     private float SPF = 1f / FPS;
     private float UNLOAD_FRAME = 2 * FPS;
     private float timeCounter = 0;
     private int counter = 0;
     private int unloadCounter = 0;
     private bool isPlaying = false;
 
     // Use this for initialization
     void Start () {
         timeCounter = SPF;
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         if (isPlaying) {
             if(timeCounter >= SPF){
                 counter++;
 
                 movieTexture.sprite = GetFrame();
                 timeCounter = timeCounter - SPF;
 
                 if(unloadCounter >= UNLOAD_FRAME){
                     Resources.UnloadUnusedAssets();
                     unloadCounter = 0;
                 }else{
                     unloadCounter++;
                 }
             }else{
                 timeCounter += Time.fixedDeltaTime;
             }
         }
     }
 
     Sprite GetFrame(){
         Sprite newSprite = Resources.Load("Movie/picture" + counter.ToString("0000"), typeof(Sprite)) as Sprite;
         return newSprite;
     }
 
     void OnMouseDown(){
         isPlaying = true;
     }
 }
 

Can somebody help me about this problem? Or anybody know a better way of doing this?