Tilemap SetTilesBlock Performance

I have a question for the Unity developers that I'd love an answer to.

How come it takes over 20 minutes to set 786,000 tiles to a tilemap via SetTilesBlock()?

All I'm doing is:

Code (CSharp):

1.  
2. int index = 0;
3. for (int x = 0; x < Size; x++)
4. {
5.     for (int y = 0; y < Size; y++)
6.     {
7.         for (int z = 0; z < Layers; z++)
8.         {
9.             Entity entity = EntityType.Grass;
10.             sprites[index++] = entity.Sprite();
11.         }
12.     }
13. }
14.  
15. BoundsInt bounds = new BoundsInt(0, 0, 0, Size, Size, Layers);
16. tileMap.SetTilesBlock(bounds, sprites);

where Size = 512, Layers = 3, and sprites references a list of EntitySprite, which derives from Unity's Tile class.
So I'm setting a class instance that inherits from Tile into a 512x512x3 area.

It's been over 20 minutes and it still hasn't finished.

I don't know how it's even possible to make something this slow...

This is on Unity 2018.1.5.

(And yes the triple for loop is pointless in this example, but that's a simplified version of what the code should be.)