Wiki page on the subject. The wiki's been going down a bit lately though, so some quick tips regarding framerate:
1: Disable temperature in the init file if you haven't already.
2: Slaughter or cage as many animals as you can to reduce pathfinding calculations for them.
3: Atom smash excess crap or otherwise purge stray junk. ("autodump destroy" is your best friend command in DFhack if you use it)
4: Really, in general pathfinding, liquids in motion, temperature, and tons of junk lying around are pretty reliable culprits for framerate issues.
5: If you have a military barracks where lots of dwarves are sparring at once, all those combat calculations can add up to a framerate squeeze.
6: Stockpiles can wreak havoc on your fps if you make gigantic ones. However, they can improve your fps if you design your fort to have them near their appropriate workshops.
7: Caverns can crush fps easily if you have multiple of them, or if you generated them with high passage density.
8: Relatedly, trees both above-ground and in the caverns can essentially grow into a hedge maze that every nearby creature needs to pathfind through.
Probably one of the most common causes for "hey why's my game so slow now" is digging out a ton of stone and not stockpiling it or walling it off, then having furnaces and workshops running constantly. The result is a depletion of all the nearby stone and the dwarves at work pathfinding into the mess of tunnels and staircases over and over, having to go deeper every time.