Here is an example that uses the effect "Color Fusion (Masked)" to create a fog of war. This example with a 4000x4000 layout in a 1024x576 window runs at over 1000 fps on my pc, when all 1600 sprites that reveal the fog are created.
1) Create your layout
2) Add a "fog of war" layer
3) Apply "Color Fusion (Masked)" as a layer effect
4) Use sprites created on the "fog of war"-layer as cutmasks to the fog
You could further optimize this example by creating/deleting the cutmasks when they get in or out of sight. You can use different shapes or even animated shapes, use a smaller or wider grid size, etc. The advantage of this method is that you don't use any additional VRAM (the layer effect always renders to screen size). Also, the layout size isn't important (CC cares more about sprites on screen, than the total number of sprites). And since the revealing is made out of sprites, you can have a vivid fog, e.g. different stages of revealing, instead of just 100%.