The short version: look
very carefully at the 'saving historical figures' line when you're doing the initial save of a newly genned world.
The long version: I jumped right into trying out the new Dorfiness of .34 with a Phoeb pack download, a large world, medium history, lots of savagery, and a 5x5 embark square. My computer is fairly bestial, so it can handle normally anti-FPS params and large embarks. But when I finished tweaking for my no tools build and got to the fortress view, FPS dropped to not twenty, not ten, not five...
Two.
Twoooooo.
FPS.
With seven dorfs and no waterfalls.
After screwing around with init.txt and d_init.txt for ages, and removing graphics packs and the whole other nine yards of elf entrails, something random occurred to me... I abandoned the fort, went back to the embark screen, and moused over a GIANT purple blotch on the embark screen I hadn't looked at very closely: an almost 15x15 embark tile area of terrifying biome. Huh.
Then I looked at legends, and I realized who had killed my FPS.
The world had over 25000 entities running around. With medium history. Most of them were undead hands flopping around abandoned battle fields. I also remembered vaguely glancing at the 'saving historical figures' number when I genned the world, and a tiny part of my brain going 'woah,' while the rest was going 'COFFEEE.'
Beware of Necromancers. They can lag death your fort before you even start.
(Note: I'm not 100% sure having a glut if entities will kill FPS, but that was the only stand-out thing about that world. I tried this again with another world, same params, and a 6x6 square, and I was immediately hopping along at 49 FPS. It's the only changed variable I could find).