Just because pathfinding is the main load on FPS does not mean that if you throw in enough things that are not pathfinding you will not end up killing FPS.
If you build enough blocks it will kill FPS. That is because every block is a tracked object, the game has to continually count every single brick in order that it can then tell you how much the total value of your fortress is. The same does not apply to the regular rocks because the game does not have to count them up for anything. But why do even need to know what the total value of your fortress is, are you intending to sell it anytime soon?
This is a problem with the game. It is totally blocked up with redundant mechanics that are not needed for anything at all. If we actually were to sell our fortress, then it would make sense to then count the total amount of blocks in your fortress because the total value of your fortress now means something. We could also force-pause the game for a while, in order to reuse processing-power for said calculations. I suppose not to be too harsh on the devs, back when they were less experienced Toady One just threw in mechanics without much through for the cost/necessity ratio until they ran into the limitations of the engine; I find no evidence that things are being developed inefficiently nowadays.
This is why adding proper squad mechanics would improve FPS. By removing active squad members from the list of regular pathfinding creatures which is not presently what happens, only the squad now has to pathfind not all it's individual members.
I see your point with squad pathing. Not individual thing. Well, pathfinding for 200 Dwarves and animal groups, even in caves when I uncovered them, never brought down alone my fps down. Not significantly. I never had visitors however. I noticed in init.txt that there could be 200+ of those. I had invasions though with 200+ invaders and their animals. That usually tended to temporarily somehow lower my fps, until my atom smasher dealt with it. I used at first "dump" quantum stockpiles though. Now I use minecart automation. Path finding has little impact on my games.
I used to have once an embark, where i created lots of 11x11 pastures to try to farm as many different species as I could. I even took upon domestication of wild animals present in my biome. That embark was a blast fun at start. However you couldn't automate pasture management of those herds. Milking and sharing jobs just refused to detect availability of proper animals spamming announcements with never ending error spam. After a while, with more bricks put in to enclose more pastures, the fps died and harassment of constant manual ordering of animals to slaughter took slowly fun out of it. Also breeding birds was problematic. You never know when to kill grown animals and close door so new could be born. Usually they just died on their own and I was lucky, if a couple survived old age to lay eggs. So, I ended that experience. Now I do alpacas, guard dogs and cats. Everything else dies on sight. Not because pathinding gives me issues, but farming animals is such an organisational pain.
Fortress value kills my fps? Not good. Maybe some optimization would be in order? Construction is what you do in DF. So more fun the less fps and fortress dies to slow fps death? Eh. Sad.
I don't mind lag from 5000 stones and 5000 logs and items made out of those. Those items get used, sold or quantum smashed anyhow in the end. I can liberate myself out of those in need. However lag from 25k bricks spent on construction jobs I can't do anything about without deconstructing those. Also construction value and quality is a totally hidden value. The constant harassment of frozen for few minutes z-stock menu when going through "blocks" entry... I would say chop those blocks down from tracked items list. Remove their value. 20k of 5 Dwarf bucks blocks is like what... 100k? Is it even significant?
Do Dwarves need to feel joy over well build bridge, workshop, trap, if player has no ability to check on those qualities to order something to be redone until it reaches master quality levels? This is so pointless, it is depressing. Maybe I should tear down all bridges and build them again until one Dwarf gets pleasure from it? Same with traps. Same with roads. And how constructing walls and floors compares to those?
What skills impact construction quality. Construction? Architecture? Masonry? Mechanics? Used items' quality? This is so obscure mechanic in this game. It is pity it also kills fps, like a vicious and ruthless slow fps death clockwork.
I can't dig underground caves because of cave adaptation and that green goo, which I can't ever get entirely cleaned from everywhere. I read the vomit, alcohol and blood pools didn't use to be a thing in DF in past, but in one expansion they got blown away without any cleaning means given to players to combat that.