There's an option in world-gen to cull some historical figure.
And the Legends Viewer can dump a ton of data into xml file.
So one (or Toady) would need to some UI to present all the legends and history, and then allow the user to select which info to "cull" and basically not track anymore, then apply that to the save file.
Other than "the world moves on data" there are some dfhackery to basically destroy objects players think are useless: blood splatters, contamination of disease etc, water flow, temperature, weather. All these small details that make DF remarkable, yet most likely ignored during fort mode game play.
What else? Pretty much any object that wants pathing: dwarfs, visitors, animals domestic and wild, invaders and hostiles, stockpile job hauling. Some of these can be handled in dfhack, but adding it all together is a pretty cumbersome task.
There are some info on the wiki to help preserve FPS hit, but there is no info to prevent FPS hits. That is the nature of the current game.
I think some tweak can be done to a world gen for a Small region, with plenty decent civ count, limited to 1 cavern layer, of about 30z-60z from surface to magma sea. A smallish embark larger than a 1x1 and smaller than 3x3. Pop cap, visitor cap, temp and weather off. Dfhack auto clean each month/season. Disciplined on use of domestic animals if at all. Atom smasher for excess items.
With the latest work order, can probably get away with having just enough booze and food for the pop and visitor cap. A tavern without a keeper. Make most inactive military with minimal uniform: robe and socks only?
There are a couple of 200+ year forts out there. What does one really want out of their DF game?
With the above setup, say an 80 pop cap, 0 visitor--can it handle 1,000 invader siege? Probably.
p.s. multi trees, that shed leaves/fruits are evil in FPS too.
p.s.2 oh, say when you have 10k+ of item, like blocks used in construction--going to the z stock menu will have a noticeable lag as the cursor pass over the blocks. So limiting number of objects perhaps also affect FPS in some way.