Cathelms was 19 years old, it was only a 1x2 but it was something stupid like 300...maybe 350 z from the height where the eagles spawned, like 100~150 z to the glacier where they hit and exploded when the constant rain of pretty blue ruphie juice knocked them out.
I ran with 20 dorfs for the first 8 or 9 years, then when updating to a new install I forgot the init limits and had to raise it to 40, then after 15 or so years I raised the cap to 90 and started doing all the room stuff there.
I would periodically flood the glacier with magma to get rid of the everpresent zombie sieges. There were eagles flying in, pathing around, and then kersploding all the time.
Never dropped under the 180(25) caps. Even at the end when I pulled the zombocalypse lever and released all the cages in the Zoombie it only dropped to 100 or so.
Most people won't be happy with just a 2x1 or 1x2 fort, I like them for specific purposes though, and I tend to build/dig vertically more than spread anyways so it doesn't change much vs having a larger embark.
As for active world causing lag, if this were the case why doesn't all the nonsense going on in adventurer mode cause the same lag?
When I get town-wide insurrections where 12,000 dorfs and goblins are all trying to move around and kill each other it gets pretty damn lagtastic, I don't know the fps because I turned it off, it's useless when adventuring to know you're at 960 fps. Besides that sort of craziness only things like sealed libraries full of scholars biting each other to death triggers the really awful lag spikes. Even full pop dark forts aren't as bad as those libraries.
Now, evil glaciers at nightfall when all the billions of zombie yetis and polar bears start roaming around? That's unbearable at times, but the solution is easy: shove something through their brain until they stop moving.