In the end draw_quad still appears to slorp down to 15 fps. And I now have the disadvantage not to be able to draw panels in front of the level.

I guess clumped prefabs also requires a lot of calculating to determine where the clumps are, and again when a tile in the prefab is destroyed. But since there are large spots of ground that are all the same, may be a solution. It's just a downside that the fps goes down when you dig more pathways in the screen... unless the pathways are bitmaps in front of the tiles... hm...

Not sure about a shader, never fancied them as they tend to make my laptop cry (no matter how simple they are), and only used premade shaders thus far. I have no idea where to start writing a shader that puts tiles on pixels of a lower scaled bitmap generated from a noise function. I guess it's like using an array and bmap_blit, but somewhat faster?