Ramps are extremely FPS friendly on the other hand. It's why the outside map, even with steep slopes and tons of ramps, doesn't cause lag. Design your fortresses with ramps instead of stairs and traffic zones and you'll see your FPS drastically improve.
Not sure what the logic is behind that proving anything. The only time the outside map is your only source of ramps or stairs is when you first embark, but a first embark is going to be fast if only because you have 7 dwarves. Not to mention no issues with caverns, manual waterfalls, stone/seeds, etc.
Edit: Grr, must learn to read entire threads before replying.
If I may be forgiven a personal plug, feel free to explore my Canyonlance map for an example of aesthetically pleasing and efficient ramp designs (link). With 7 years of history, 50 dwarves and close to 20,000 items I still get 100FPS.
Also, I'm not sure that a 3x3 with 50 dwarves running at 100 FPS is particularly noteworthy.
I doubt I have 20,000 items (no idea how to check that besides manually tallying the lists in stocks), but my five year fort of 80 dwarves runs at 100 FPS, baring ambushes, despite my staircase stack hitting through to the first cavern layer. I even have temperature turned on and an active waterfall in my dining area (15x15 square with 4 3x3 "splash" areas); however, eventually dumps up into an aquifer, which I think is a lot more FPS-saving than my previous constructions pumping water through fortifications off the map. They also have minimal flow areas.
However, I will say I like the empty central pillar format, with quadrants being designated for the appropriate industries. Might be a little less inefficient than staircase stacks in terms of steps taken by dwarves to get around, but probably a lot easier to design than my ~16x16 cubes of industry. Will also work for caravans once wagons are back in.