Changing Camera.Rect in coroutine causes huge fps drops

Hi, so I have two camera displaying to the screen. One camera shows the 3D player and the other shows a 3D board.
When the player triggers something I want one camera to expand the cover the whole screen and the other camera to move off screen. I have this code that works but it seems that quickly changing the Camera.Rect each frame cause a huge fps drop (150 down to 1-3fps).
I tried doing this in update but had the same fps drops.

How can I smoothly expand the camera across the whole screen without causing performance issues?

Code (CSharp):

1. public Camera boardCamera;
2.     public Camera worldCamera;
3.     public float transitionTime = 2f;
4.  
5.     Vector4 boardCameraStart;
6.     Vector4 worldCameraStart;
7.  
8.  
9.     void Awake()
10.     {
11.         boardCamera = boardCamera.GetComponent<Camera>();
12.         worldCamera = worldCamera.GetComponent<Camera>();
13.     }
14.  
15.     public void FullScreenBoardCall()
16.     {
17.         GetCurrentCameraRects();
18.         StartCoroutine(ChangeCameras(new Vector4(0f, 0f, 1f, 1f), new Vector4(1f, 0f, 0.5f, 1f)));
19.     }
20.  
21.     public IEnumerator ChangeCameras(Vector4 boardCameraEnd, Vector4 worldCameraEnd)
22.     {
23.         float startTime = Time.time;
24.  
25.         Vector4 boardCameraStart = new Vector4(boardCamera.rect.x, boardCamera.rect.y, boardCamera.rect.width, boardCamera.rect.height);
26.         Vector4 worldCameraStart = new Vector4(worldCamera.rect.x, worldCamera.rect.y, worldCamera.rect.width, worldCamera.rect.height);
27.  
28.         while(Time.time < startTime + transitionTime)
29.         {
30.             Vector4 worldRect = Vector4.Lerp(worldCameraStart, worldCameraEnd, (Time.time - startTime)/transitionTime);
31.             Vector4 boardRect = Vector4.Lerp(boardCameraStart, boardCameraEnd, (Time.time - startTime)/transitionTime);
32.             worldCamera.rect = new Rect(worldRect.x, worldRect.y, worldRect.z, worldRect.w);
33.             boardCamera.rect = new Rect(boardRect.x, boardRect.y, boardRect.z, boardRect.w);
34.             yield return null;
35.         }
36.         worldCamera.rect = new Rect(worldCameraEnd.x, worldCameraEnd.y, worldCameraEnd.z, worldCameraEnd.w);
37.         boardCamera.rect = new Rect(boardCameraEnd.x, boardCameraEnd.y, boardCameraEnd.z, boardCameraEnd.w);
38.  
39.     }
40.  
41.     void GetCurrentCameraRects()
42.     {
43.         boardCameraStart = new Vector4(boardCamera.rect.x, boardCamera.rect.y, boardCamera.rect.width, boardCamera.rect.height);
44.         worldCameraStart = new Vector4(worldCamera.rect.x, worldCamera.rect.y, worldCamera.rect.width, worldCamera.rect.height);
45.     }