Well, i decided to create a new fortress in a Rocky waste (fireclay ground, no trees or other vegetation) and the improvements wasnt very noticable, FPS kept dropping below 60 quite soon when watching the above ground.
Then I dumped that fort and created new one in moderate vegetation location (some trees, some plants and other greenery) and decided to try a new thing -- I didnt build anything except roads and floors on the above groud (total flat) level, because previously i've been building towers, castles and other stuff (mainly to avoid sun sickness and for above ground crops). I believe, that this helped quite a lot in general (underground FPS can be even 90 for a quite long period of time, above ground drops below 60 only in siege or wood gathering. And i dont think i changed anything else, have same fort design, digged the same amount of rock.
Still the difference between underground and above ground thing is really annoying and 30 FPS is quite a big deal for me personally. While DF is awesome and I wish the authors the best, I really dont understand the development decisions. If I understood correctly, there are going to be some new features like e.g. taverns and libraries, which is cool, but what good it is, if the game gets unplayable after a few years of running a fort and adding new stuff surely wont help with this. And this problem with viewing the different parts of the game area looks more like problem with really inefficient code (rendering?), which should't be that hard to fix (and the impact would be great I believe). While the performance issues related to game complexity and scaling might not be solved even with paralel computation or better hardware, little consolidation of the code shouldnt be an unattainable goal.