...In comparison, Goatmaan's nearly 1000-dwarf fort had one in-game day take 7 minutes and 12 seconds, iirc.
Of course, playing is more about designating, but still, wtf. Checking the save on 43.05 vanilla (hope no save-breaking mods)....
...Loading it up takes something like 1-2 minutes - this is the largest fortress in disk space and RAM I've seen, taking up 1,9 GB RSS and 2,5 VM-Size when loaded.
FPS seems initially much greater than what you reported; 17-20 when loading the fort, but can certainly be improved. Takes me 5 minutes to reach 6th Granite on opening screen. My first thought is about somebody setting doors to be pet-impassable...
Well, rooms are huge. Huge pastures here, though beautiful microcline stars.
I don't need to speak about main fort dug out by "make gloomdiamonds great" efforts, but there are also yuge unwalled quarries on z99, for instance.
And..Aha! There are lot of pet-impassable(tightly closed) doors with pets pastured inside.
Mostly on z124, such as Elk birds:
Rhinos
A whole horde of cats
Hm, what else...Ah yeah. Max™ has been complaining in FotF that these rampways
Make worldgen forts have the lag of cities ten times their size.
Changing all the doors of the above to forbidden pet-passable helped boost FPS to for me to 22. Less than expected, hm.
There are also no traffic designations in the hallways, so that could also improve FPS by another 10-20%.
I guess there's also lot of animals(160, 19 caged/chained) and guests/others(129, 29 caged). All in all, could be worse.
Including, funnily, a flashing elf with Great skill in lying (and bunch of others, but none legendary):
Well, I hope this helps.