Very Low FPS rendering only meshes

I've been working on a game that generates/renders a fractal terrain at runtime using meshes. I was doing this on very small power of 2 sized maps and everything was fine. As soon as I increased the size to 128x128 the application became a slide show.

I culled my project down to a bare bones test case. I was trying to find why the performance is so low with only 32k triangles, but without luck. I'm hoping I've missed something simple.

Admittedly I have a pretty old machine. it's a dual core 3.1ghz with 4gb of ram and directx 9. It does have an nvidia gtx460 gpu however.

I created a new project with 3 objects and 1 script. The default camera, an empty game object with my script attached to it, and a third empty game object. The third one is a prefab with a mesh renderer, mesh collider, and mesh filter attached. I disabled vsync in the project settings.

Here's my lone script:

 using UnityEngine;
 using System.Collections;
 
 public class gen : MonoBehaviour {
 
     public Transform mt;
 
     private Mesh mesh;
     private MeshFilter mf;
     private MeshCollider mc;
     private MeshRenderer mr;
 
     private int size = 129;
     
     void Start () 
     {
         CreatePlaneMesh();
 
         for (int x=0; x<size; x++) 
         {
             for(int z=0; z<size; z++)
             {
                 Transform tr = Instantiate (mt, new Vector3 (x*3f, 0f, z*3f), Quaternion.identity) as Transform;
             }
         }
     }
 
     public void CreatePlaneMesh()
     {
         mesh = new Mesh ();
 
         mf = mt.GetComponent<MeshFilter> ();
         mc = mt.GetComponent<MeshCollider> ();
         mr = mt.GetComponent<MeshRenderer> ();
         
         mf.mesh = mesh;
         
         Vector3[] vertices = new Vector3[]
         {
             new Vector3( 1, 0,  1),
             new Vector3( 1, 0, -1),
             new Vector3(-1, 0,  1),
             new Vector3(-1, 0, -1),
         };
         
         Vector2[] uv = new Vector2[]
         {
             new Vector2(1, 1),
             new Vector2(1, 0),
             new Vector2(0, 1),
             new Vector2(0, 0),
         };
         
         int[] triangles = new int[]
         {
             0, 1, 2,
             2, 1, 3,
         };
 
         Vector3[] normals = new Vector3[]
         {
             new Vector3( 0, 1,  0),
             new Vector3( 0, 1,  0),
             new Vector3( 0, 1,  0),
             new Vector3( 0, 1,  0),
         };
         
         mesh.vertices = vertices;
         mesh.uv = uv;
         mesh.triangles = triangles;
         mesh.normals = normals;
         mesh.RecalculateNormals ();
 
         mc.sharedMesh = mesh;
     }
 }

No update methods. Nothing else in the project. When I run it in the webplayer I get sub 20 frames per second. That seems very low when rendering <25k triangles on screen. The docs state you should keep your triangle count under 3m on a desktop, so I'm really confused. It pegs one of my cpu cores at 100% so I'm guessing this is CPU limited instead of GPU limited.

Is there some simple gotcha I've overlooked here? I plan to cull distant/hidden objects in the future, but I'm concerned about the low FPS.