Procedurally generated mesh performance

Hello. I am generating a hexagon mesh procedurally, and putting it in a game object, like so:

 GameObject hex = new GameObject ();
         hex.AddComponent<MeshRenderer> ();
         hex.GetComponent<MeshRenderer> ().castShadows = false;
         hex.GetComponent<MeshRenderer> ().receiveShadows = false;
         
         hex.AddComponent<MeshFilter> ();
         hex.GetComponent<MeshFilter> ().mesh = GenerateHexagonMesh ();
         //hex.renderer.sharedMaterial = mat;
         hex.isStatic = true;

I am instantiating the object about 400 times, without any performance issues. However, if I set a material to the object (tested several), the performance drops to the point that Unity just stops responding. If I assign a material, only 10 or so objects can be instantiated before the performance gets too bad.

Any tips to what might be going wrong here? Thanks

edit: this is how i generate the mesh:

 private Mesh GenerateHexagonalMesh (float hexRadius)
     {
         Mesh mesh = new Mesh ();
  
         Vector3 vertPos = new Vector3 (0, 0, hexRadius);
         Vector3[] vertices = new Vector3[7];
      
         Quaternion rot = Quaternion.Euler (0, 60, 0); //360/6 = 60
             
         vertices [0] = new Vector3 (0, 0, 0);
         for (int i=0; i< 6; i++) {
             
             vertPos = rot * vertPos;
             vertices [i + 1] = vertPos;
         }
  
     
         mesh.vertices = vertices;
  
         int[] tri = new int[]{
             0, 1, 2,
             0, 2, 3,
             0, 3, 4,
             0, 4, 5,
             0, 5, 6,
             0,6,1};
  
         mesh.triangles = tri;
  
         Vector3[] normals = new Vector3[7];
  
         normals [0] = Vector3.up;
         normals [1] = Vector3.up;
         normals [2] = Vector3.up;
         normals [3] = Vector3.up;
         normals [4] = Vector3.up;
         normals [5] = Vector3.up;
         normals [6] = Vector3.up;
  
         mesh.normals = normals;
         //mesh.RecalculateNormals ();
         mesh.RecalculateBounds ();
         mesh.Optimize ();
         
         
         return mesh;
     }