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?

Try not loading a single frame each update. Instead, load all your frames into memory in Start() and loop through them in Update().

Or make an animation prefab for your animation and use that + the animator.