lag death is the most common form of fortress death and it's not really known what happens. There are many little things that add up i guess.
DF is super processor intensive. It tracks all of things in your fortress, moves all the liquids, paths all of the creatures, and keeps track of all the things happening in the larger world. all of these things become harder as the fortress gets older.
A lag resistant embark would be on a tiny no-history world, have no liquids, no trees, basically a featureless wasteland. You'd try to minimize units ever leaving your map to reduce world influence. If the embark was 2x2, it'd probably survive 20 years before lagging out.
There's also the G_FPS trick, which can help fortresses run a lot smoother at the cost of graphical stuttering. We could probably use this to run Flamehorses another few years i guess before it got too bad now that i think about it.
All that being said, it doesn't seem like fortress age should matter as much as it does. who knows. I doubt the necromancy had a significant impact on lag. The place was doomed to lag death anyway, being a world full of things that happen, and a map with trees, water, magma, caverns, and multi level surface.
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a last bit of science.
destroyed practically all objects, 19097 of them.
Volcano FPS 17
slayed all but one dwarf (BARON VON QUELL SURVIVES!)
Volcano FPS 27
i can get 100fps underground, but still only 27 over the volcano.
i couldn't tell what was happening in the volcano so i turned off multilayer rendering.
Volcano FPS 100
huh.