By my observations neither process at REALTIME nor binding dwarf fortress to a single core or processor (via affinity of process) bring any fps.
2x2 to 5 embarks are infatigable fps if you don't dig too deep.
Less z-layers pretty much guarantees 150+ fps (I play capped at 120 fps) no matter how big your fort is to a certain point. Destroying stone, clothes and stuff (goblinite for example) in DAS or magma doesn't seem to help improve FPS, sadly.
2x2 embark with 400 dwarves counting babies and children gives a steady 120 fps no matter what I do with stone levels. It all goes to 75ish if I have any water. I heave also read somewhere that disabling weather and especially temperature doubles your fps, but what kind of fortress would it be without proper magma.
If you have seen your share of stuff "digging too deep" dftubefill guarantees around 1/5 to 5/5 fps when you hit that stuff. I really advise you to go ahead and use it for your fort. For the sake of efficiency on my old Macbook I never use any hallways, everything has to be reachable both by z above and below and by walking. It seems to help smooth things a bit when I designate some mass dumping of stones at least. I have a lot of stairs this way and many people here don't like stairs so I'll just leave it here as a partial solution. I use ramps to get my caravans down to z-3 where the trade depot is.
There appears to be a massdump utility in dfhack package that allows for instant destroying of stuff you dump, this might also help you out.
Going by numbers my biggest fort is at 300 dwarves with children in a Tundra biome with no goblins and I never have fps less than 120 even with my magma forges running. It dips a little if I switch all dwarves to hauling and back to doing their jobs, though. It's a 3x3 embark, I can provide you with a save if you like, the dwaves there are named after guys in my former guild in World of Warcraft (some of them are currently top1 by achievement points in World of Warcraft, like my baron (or duke something) Korealstraz), we used to play together in the Enrage guild on Borean Tundra-EU.