Hey, I'm not sure who theorized this (retro I think?) but they said that {b}>{C}>{f}ing everything actually increases FPS because it lessons the amount of tiles that the game has to load out of world information and generation. Reason being, put a {b}>{o} on something then {k} it, and you'll see the natural floor is still listed under the {b}>{o}, but when using {b}>{C}>{f}, it only displays the {b}>{C}>{f}, and doesn't load the natural world tile under it anymore.
It actually stops recognizing the natural world tile is even there now. I was building a fort once where I was obsidian casting the ocean, after doing about two levels I started building some houses on top. I messed up and had to remove some {b}>{C}>{w}'s. When I removed them, the natural floor under them was not obsidian, it was red sand or silty loam or some such. The game had no idea what the natural tile was (wasn't stored in memory), and had to pull it from world gen (which I guess for that spot would have been red sand if it had been above the ocean).
TL;DR
Putting {b}>{C}>{f} over everything reduces the amount of memory DF needs to run, and increases FPS.
Multiple level pathfinding does slow things down. I'm not sure how pathfinding algorithms work, but I'm pretty sure having multiple levels (with dwarves on them) forces DF to have to load more levels into its memory, hence slowing things down a bit. Don't quote me on any of that.