I can play a larger area so thats wonderful so one question...
How large are the caverns?
Erm, massive. Generated the world with maximum Openness and minimum Passage Density, so they're literally cavernous. As mentioned previously, the third cavern layer is also extremely tall.
If you just started playing, and are experiencing significant lag, please see the second edit of my first post - the second cavern layer has a leaky section that'll require patching to prevent early FPS death. Sadly, this is located directly above the demon temple, so if your embark includes that spot you'll have to deal with it one way or another, I suspect.
Alright, here's what I'm going to do:
1- Re-embark. Same location, just a completely clean start better tuned to what I know is coming.
2- Dig straight down to the first cavern layer. There's a magma pool there, and it's connected to the second cavern layer by a passage. This will become my dwarves' front yard.
3- Quarry stone wherever necessary to build a pseudo-aboveground cluster of huts near the magma pool, while at the same time converting it into a
sweatshop slave labor camp center of industry for my settlement.
4- Dig further. The goal is to begin acquiring the adamantine as quickly as possible (seriously, it's available before the iron is), and with its help, clear out the demon temple so that it can be colonized. Dwarves of sufficient merit will be given rooms in the temple.
5- Embrace glorious warfare in the true
Sontaran Dwarven fashion! It may be ambitious, but I intend to conquer hell this time!