thing about DF is that unlike TES, when its FPS goes down, its because the game takes longer to calculate everythings movements for that frame, so weather your playing at 1 fps or 100, the game still renders out each frame.
so while it may take 100 seconds at 1 fps to equal 1 second of gameplay at 100 fps, its really a matter of patience in DF.
my current comp is a 5 year old laptop, and it was mediocre when it was new, so it chokes on even moderate graphics (oddly enough though it plays KSP at 40-60 fps, even with a +200 parts ship on screen)
im used to playing games at 8-15 fps, lower than that and things just get too choppy, but since df has such simple graphics, its really just a matter of patience as to when your fort dies of FPS death.
you might want to try DF hack, make a tunnel to the blue metal that dosnt open into the caverns, and then use df hack to hide all non-accesable locations, that should give you some fps back.
and if you have a clothing industry, you might want to remove all storage from your dwarves bedrooms and make their rooms a refuse stockpile that dosnt accept anything, your dwarves will put their worn out clothing on their floors, and the refuse stockpile will make the clothes disintegrate even though the stockpile dosnt accept clothing. you make it so it dosnt accept anything so that your dwarves dont store garbage in their rooms. that trick really helped me out on my fort where i had so much worn out clothes that i could buy out every caravan and donate all the clothing that they could carry off with them and still have way too much clothing (that was actually my last fortress that i made with LNP and all the wonderful plugins like automation before df started updating faster than LNP could, though i hear df hack has a beta for 0.43.05 so lnp should finally be up to the latest version in a month or two)
also would like to make note, i run DF on both windows and linux, i have both on separate HDDs because some games run faster on linux, and some dont have a linux version, so i like to keep both.
ive never really noticed much of a diffrent in DFs speed between the two when in the same fort, but if your computer has low ram, you should definitely try linux, its a lot lighter on your ram and CPU than windows.
i personally use ubuntu.