How do you use submeshes correctly?

Hi! I want to generate a tile map in unity and came up with two implementations:

1. using a gameobject for each tile

2. using MeshFilter, MeshRenderer: Create a submeshes for each tile

I expected that the version using a mesh would be faster than the one using gameobjects, but this was not the case - so I am wondering if I did it correctly (I was reading a tutorial where a big texture was generated by setting pixels and applied to the whole mesh - the author mentioned that using individual vertices for each tile would be more efficient and that it would be easy to apply different textures to each tile - I thought he meant submeshes):

I am generating my tiles using two loops:

 Vector3[] vertices         = new Vector3[6*hexagonCount];
 Vector2[] uv                = new Vector2[6*hexagonCount];
 List<List<int>> triangles  = new List<List<int>>(hexagonCount);
 for(int y=0; y<tileNumberY; ++y){
   for(int x=0; x<tileNumberX; ++x){
     generateHexMesh(ref vertices, ref uv, ref triangles, index, posX, posY, posZ);
   } 
 }

These loops fill my vertices, uv and triangles list/arrays. After that I do the following:

 Material[] materials       = new Material[hexagonCount];
 this.mFilter.mesh.vertices = vertices;  //add our vertices to the mesh
 this.mFilter.mesh.uv       = uv;     //add out UV coordinates to the mesh
 this.mFilter.renderer.materials = materials;
 
 for(int i=0; i<hexagonCount; ++i){
   int[] temp = triangles[i].ToArray();
   this.mFilter.mesh.SetTriangles(temp, i);
   this.mRenderer.materials[i] = material;
 }
 this.mFilter.mesh.RecalculateBounds();     //recalcualte the dimensions of our mesh
 this.mFilter.mesh.RecalculateNormals();

My code generates the desired Hexagons, but the framerate drops to 5 FPS when running it for 100x100 tiles, while the gameobject version runs at 300 FPS. Did I do something wrong or are submeshed not meant to be used in such ways? How are submeshes used correctly to be able to assign different textures to those tiles?

EDIT: Updated code according to robertbu's suggestions

 this.mFilter.mesh.vertices = vertices;  //add our vertices to the mesh
 this.mFilter.mesh.uv       = uv;     //add out UV coordinates to the mesh
 
 for(int i=0; i<hexagonCount; ++i){
   this.mFilter.mesh.SetTriangles(triangles[i].ToArray(), i);
 }
 this.mRenderer.material = material;
 this.mFilter.mesh.RecalculateBounds();     //recalcualte the dimensions of our mesh
 this.mFilter.mesh.RecalculateNormals();

UVS used for each hexagon:

 // generate uv
     Vector2[] uv = new Vector2[]{
       new Vector2(0,0.25f),
       new Vector2(0,0.75f),
       new Vector2(0.5f,1),            
       new Vector2(1,0.75f),
       new Vector2(1,0.25f),
       new Vector2(0.5f,0),    
     };