Modifying terrain at runtime LAGS

Hey guys, i'm trying to modify the terrain at runtime by using the build-in SetHeights function. It works perfect but it drops the FPS from 70-90 to 2-3 even if i try to modify 1 point. I tried at very small terrains and at normal terrains, there was no difference.

Here is the script i'm using :

Code (CSharp):

1.  public Terrain currentTerrain;
2.  
3.     int xRes;
4.     int zRes;
5.     float[,] heights;
6.     TerrainData tData;
7.  
8.     void Start()
9.     {
10.         tData = currentTerrain.terrainData;
11.         xRes = tData.heightmapWidth;
12.         zRes = tData.heightmapHeight;
13.         heights = tData.GetHeights(0, 0, xRes, zRes);
14.     }
15.  
16. void Update()
17.     {
18.         if (Input.GetKeyDown(KeyCode.Mouse0))
19.         {
20.             RaycastHit hit;
21.             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
22.             if (Physics.Raycast(ray, out hit))
23.             {
24.                 ModifyTerrain(hit.point);
25.             }
26.         }
27. }
28.  
29. private void ModifyTerrain(Vector3 point)
30.     {
31.         int mouseX = (int)((point.x / tData.size.x) * xRes);
32.         int mouseZ = (int)((point.z / tData.size.z) * zRes);
33.  
34.         float y = heights[mouseX, mouseZ];
35.         float[,] modifiedHeights = new float[1, 1];
36.         y += 0.005f;
37.         for (int x = -6; x <= 6; x++)
38.         {
39.             for (int z = -6; z <= 6; z++)
40.             {
41.                 if ((mouseZ + z) <= tData.heightmapHeight)
42.                 {
43.                     modifiedHeights[0, 0] = y;
44.                     heights[mouseX, mouseZ + z] = y;
45.                     tData.SetHeights(mouseX + x, mouseZ + z, modifiedHeights);
46.                 }
47.             }
48.         }

So is there any way to prevent the lag or any other way to modify the terrain ?