Modifying Terrain Splats Dynamically

My aim is to paint the terrain a run time (specifically load-time). While I do have a method for this, as shown below in its most rough form, it takes an EXTREMELY long time to paint even a small square of the terrain.

Could anyone point out a superior method for this?

Code below:

  public void Paint()
     {
         TerrainData td = Terrain.activeTerrain.terrainData;
 
         float[,,] element = new float[1, 1, 8];
 
         for (int y = 0; y < 100; y++)
         {
             for (int x = 0; x < 100; x++)
             {
                 int rand = Random.Range(0, 4);
 
                 if (rand == 0)
                 {
                     element[0, 0, 0] = 0; 
                     element[0, 0, 1] = 0; 
                     element[0, 0, 2] = 0;
                     element[0, 0, 3] = 0; 
                     element[0, 0, 4] = 0;
                     element[0, 0, 5] = 1; 
                     element[0, 0, 6] = 0;
                     element[0, 0, 7] = 0;
                 }
                 else
                     if (rand == 1)
                     {
                         element[0, 0, 0] = 0; 
                         element[0, 0, 1] = 0; 
                         element[0, 0, 2] = 0; 
                         element[0, 0, 3] = 0; 
                         element[0, 0, 4] = 0; 
                         element[0, 0, 5] = 0;
                         element[0, 0, 6] = 0;
                         element[0, 0, 7] = 0;
                     }
                     else
                         if (rand == 2)
                         {
                             element[0, 0, 0] = 0;
                             element[0, 0, 1] = 0; 
                             element[0, 0, 2] = 0; 
                             element[0, 0, 3] = 0; 
                             element[0, 0, 4] = 0; 
                             element[0, 0, 5] = 0;
                             element[0, 0, 6] = 1; 
                             element[0, 0, 7] = 0; 
                         }
                         else
                             if (rand == 3)
                             {
                                 element[0, 0, 0] = 0;
                                 element[0, 0, 1] = 0;
                                 element[0, 0, 2] = 0;
                                 element[0, 0, 3] = 0;
                                 element[0, 0, 4] = 0;
                                 element[0, 0, 5] = 0;
                                 element[0, 0, 6] = 0;
                                 element[0, 0, 7] = 1;
                             }
 
                 td.SetAlphamaps(x, y, element); // update only the selected 
             }
         }
     }

My poor attempt is a rough amalgamation of various methods found across the internet. All it does it, within the small defined 100x100 square of terrain, apply a random splat to each "square".

If anyone could help, I would be extremely grateful