I love me some 5x3 embark areas; but as soon as I really cut into the bedrock; I quickly drop from 100/50 FPS to 25/15 FPS (exaggeration alert!). In fact, the most common cause of fortress abandonment for me isn't Goblin Sieges, it isn't lava-related Fun, it's not tantrum spirals, it's terrible lag, even when running a population cap of 20.
From what I hear; it's more or less accepted that in the current version, the main culprit is mining and loose stone; and even turning all of the stone into constructions doesn't seem to help much.
Then again; building a playable fortress while staying squarely inside the three or four levels of soil on a "very deep soil" embark seems very undwarvely…
So how can I undertake massive digging projects and avoid terribletastic lag?