I'm sure someone will correct me if I've forgotten something here, but I'm fairly certain that there are only four areas of Dwarf Fortress that cause considerable FPS drops.
- Creature Pathing and Decision Making
- Fluid Dynamics
- Heat Calculation
- Weather
I can't think of any other parts of the game that can come close in comparison to the amount of time these parts of the simulation use. They're the heavy numbers behind everything, after all.
So, if you limit your pop-cap to 20, ensure there's no large water bodies, turn off all the cavern layers and underground features, and switch off weather for good measure... You'll find a fort like this would take much
much longer to slow down at all.
But even at this hyper optimized super fast fort, it would
eventually slow down, as you modify the environment and accrue dead creatures and such.
Trying to make a single game last forever is never going to work, due to the nature of forever.
Basically Dwarf Fortress can either be played with everything switched on for a short while, or with everything switched off for a long while. Either way there is a very real upper limit the length of time you can run a single fortress for, and that's not in any way a fault of the game, but rather fault of the universe our computers exist in.
In fact, even the most theoretically ideal multi-threaded environment could be brought to a very painful halt when the entirety of the goblin race decides to invade.
(Although, in practice it would likely suffer a more boring slowdown, as hundreds of dwarves interact with a limited set of items, grinding everything to a halt in a hideous mess of memory locks.)
Alternatively skip locks and enjoy the !!fun!! of unsafe threading.