set local position unity cost huge time

I profile the NGUI code, I found setLocalPosition cost over 5ms when using scrollView.
But when I create a empty Test script like this:

using UnityEngine;
using System.Collections;

public class Test : MonoBehaviour {

// Use this for initialization
void Start () {

}

// Update is called once per frame
void Update () {
transform.localPosition += Vector3.one;
}
}
and add lots of child and parent, but setLocalPosition always cost less than 0.05ms,Why?

this seems like it would be a question for the guys over at ngui.