Hi!
For ILMBIN lots of cubes are to be expected, however at around a 25x25 square of cubes lags a LOT.
https://playground.babylonjs.com/#D1Z1VL#15
When you load the page it has a 16x16 square. Change the variable “sizeofplatform” at line 176 to, well, change the size of the platform.
Any help nice. I made the change to use instances instead of individual cubes. It added around 5 fps.
If your blocks won’t move I suggest to use instances AND call scene.freezeActiveMeshes() when all the blocks are ready
will scene.freezeActiveMeshes() allow me to place a block, and then move it?
Just tried it and it won’t work because this project is using my physics controlled camera. The player is a mesh.
Depending on how you want to architecture it but you could have two scenes with the second one set with autoClear=false and autoClearDepthAndStencil = false
Then you can have your optimized ground on scene 1 and everything else on scene 2
You can unfreezeActiveMeshes to move a mesh and then freeze the scene back when needed
So basically:
scene 1:
every block
scene 2:
autoClear = false; and autoClearDepthAndStencil = false; and the player
correct
A 10x10x10 cube. 12 fps lol 
https://playground.babylonjs.com/#D1Z1VL#16
check with freezeActiveMeshes 
New Discovery!
Going back to the playground linked in my last reply, I get 12 fps with a 10x10x10 cube. I comment out lines 181-189. This removes the 1000 physics impostors for all the cubes in the cube. I get 40 fps when looking around, 50 when still. and 60 when looking away. Finding a solution to add and remove physics impostors might work. Only the cubes nearest the player should have physics impostors.
Oh, and the 10x10x10 cube with freezeActiveMeshes didn’t help much. I added two functions, freeze and unfreeze. freeze was called when not moving the camera and when not pressing a key(moving around). unfreeze was called when moving and looking around. I noticed the cubes disappearing and reappearing, and I don’t think that it would be a good long term solution.
This kinda works ;/
https://playground.babylonjs.com/#D1Z1VL#17
The freeze should definitely largely improve the perf but only when objects are not physics. They must be static
This works pretty ok. around 10 fps
https://playground.babylonjs.com/#D1Z1VL#21