Interesting. I can manage a smooth fps with 150-odd dwarfs, rotting corpses, a caravan, and way too many pets with one revealed cavern and one partly revealed. Though my game dropped down to about 2 fps for several irl hours when i had about 90 dorfs, no caverns, fewer pets, and no water drainage. (also why doesn't the water drainage of entire flooded z-levels affect my fps?!)
It depends on what was going on in your fort at the time.
FPS drops happen because the game needs to do lots of calculations at a time, and those calculations take time. However not all calculations are equal, some are much more expensive than others. Pets are generally not much of problem because they just roam around aimlessly most of the time. Similarly the mere existence of an area isn't going to have a huge impact.
What does affect performance is pathfinding. It's very likely that Dwarf Fortress uses A* for pathfinding. It's stupidly fast, so it only really has a major impact when there are hundreds of entities all needing to path find (say a siege). That's only true if there is a path to begin with, however. You see, if there's no way to get from A to B, then it doesn't matter what algorithm you use, you'll have to search the entire area you
can reach until you determine that it's unreachable. Now, DF probably has some caching around this, so locking a door doesn't instantly tank performance, but there's always limitations. Without more understanding of how DF works at that level, I can't even reasonably speculate.