I would try a couple of things:
+ atom-smash all junk (refuse, thread from butchering...)
+ sell things that aren't directly beneficial (crafts, surplus food, cloth, seeds...)
+ lock all animals into their own rooms if possible (dogs, pigs...)
+ wall off areas you no longer need access to (mines, unused workshop areas, caverns...)
+ forbid items you don't need anytime soon (misc. metal bars, surplus wood, stone...)
I'm not gonna lie, it won't do much. Once your FPS go down it's very hard to bring them back up. But if you want to try anyway you can use DFhack tools to make the process bearable. Maybe you can start by butchering some animals. Go into your stock menu and mark all trash for dumping. Remains, corpses, cartridge, shells, thread... All those things that accumulate in the hundreds over the years. Keep a couple for moods, use DFhacks "autodump" to smash the rest.
Next dump all the surplus stuff you have and lock it away. You can use "cleanowned scattered" and "clean all item" to dump scattered sloths and clean the map from bloodspatter, leaves, vomit, dirt...
Areas like your mined-out workshop level, the caverns and the surface can cause some slowdown if they are open. So pull up the bridges and lock the doors.
It's best to do all these things on a regular basis when you have some idle haulers. In my old fort I atom-smashed everything I didn't need and kept the place lean and mean. That fort went to 17 years and was still playable. Limiting your dofs to 100 would be another great way to keep FPS up. That's enough to get proper invasions and everything but it's not quite as hard on the FPS. You won't get the mountainhome with 100 dwarfs though afaik.
Good luck!