Why Unity freezes while creating Meshes procedurally?

I'm creating a mesh procedurally with around 10000 vertices, the problem is that when I start the mesh generation it freezes for a few moments, I used the time method to get the number of frames that past but it appears that not a single one past, then the mesh is finished, but I dont want that the program freeze each time that I generate a new mesh, 'cause I'm doing it constantly.

The arrays fills automatically before creating the mesh, but just filling them without sending them to the mesh there's no lag, so I don't think the problem is there.

Also, if ,after generating the mesh, a method move all the vertices around there's no lag, so the manipulation of the mesh appears to be ok too, anyone has a clue of why is this happening?.

Here's the codo so you can take a look:

     public void Build(    GameObject GObject,
                         List<Vector3>    chunkPosition,
                         List<Vector3>    chunkRotation,
                         List<int>        chunkSides,
                         List<int>        chunkSpace){
         float time1,time2,timeResult;
         time1 = Time.time;            
         int    arraySize      = 0,
             index          =    0,
             sidesToRender = 0,
             tama          =    chunkBlockID.Count;
         
         MeshFilter    meshFilter    = GObject.GetComponent<MeshFilter>();
         Mesh         mesh        = meshFilter.mesh;
 
         mesh.Clear();
         
     //Each position in each list represents information of a cube that occupies that place
     //chunkSpace represent the number of triangles of the cube that are to be rendered

         for (int i = 0; i < tama; i++)
             arraySize += chunkSpace[i];
         
         Vector3[] vertices = new Vector3[arraySize];
         Vector2[] uv       = new Vector2[arraySize];
         int[] triangles    = new int[arraySize];
         Vector3 Temp       = new Vector3(0,0,0);
 
         ////ChunkSides contains six glags, one per side, so this cycle decides the orientation of each triangle
         for (int i = 0; i < tama; i++){//Todos los posibles bloques

             sidesToRender = chunkSides[i];
             for(int j = 0,selector  = 1,Kstart  = 0,Kend  = 6;
                     j < 6;
                     j++  ,selector *= 2,Kstart += 6,Kend += 6){
                 //If the flag of a side is on, then add the respective rendering parts
                 if((sidesToRender&selector) != 0)
                     for(int k = Kstart; k < Kend; k++, index++){
                         vertices[index] = Shapes.CubeVx[k]+Temp;
                         triangles[index]= index;
                         uv[index]        = Shapes.CubeUV[k];
                     }
             }
         }
         mesh.vertices     = vertices;
         mesh.triangles    = triangles;
 
         mesh.uv    = uv;
         mesh.RecalculateNormals();
         mesh.RecalculateBounds();
         mesh.Optimize();
             time2 = Time.time;
             timeResult = time2-time1;
             print(timeResult);
             //Get the total time elapsed (it always gives me 0)
     }