Using rigidbody.position is much faster than transform.position

Hi,

I just did some tests and find out that set position through rigidbody.position is much faster than transform.position. Here are the results, 10M loops:
When gameObject has rigidbody:

transform.position = Vector3.zero; // 5.7s
thisTransform.position = Vector3.zero; // 4.7s

rigidbody.position = Vector3.zero; // 2.9s
thisRigidbody.position = Vector3.zero; // 1.3s


When gameObject doesn't has rigidbody:

transform.position = Vector3.zero; // 4.4s
thisTransform.position = Vector3.zero; // 3.4s

I don't have any mobile device to test. All are tested on x86 laptop, Unity 4.2.2.
It seems attach a rigidbody to gameobject and set position from it is much faster than set transform position (1.3s vs 3.4s). Why is there a significant different, and is setting position through rigidbody a good method?

Here is my code:

Code (csharp):

1.  
2. using UnityEngine;
3. using System.Collections;
4. using System.Diagnostics;
5.  
6. public class test : MonoBehaviour {
7.    
8.     Transform thisTransform;
9.    
10.     Rigidbody thisRigidbody;
11.    
12.     Stopwatch timer = new Stopwatch();
13.    
14.     // Use this for initialization
15.     void Start () {
16.        
17.         thisTransform = transform;
18.        
19.         thisRigidbody = rigidbody;
20.        
21.         timer.Start();
22.        
23.         for (int i = 0; i < 10000000; i++) {
24.             thisRigidbody.position = Vector3.zero;
25.  
26.             //rigidbody.position = Vector3.zero;
27.  
28.             //transform.position = Vector3.zero;
29.             //thisTransform.position = Vector3.zero;
30.         }
31.        
32.         timer.Stop();
33.         print(timer.Elapsed);
34.     }
35. }