A few things:
1)My game usually sits at around 180 and up (I've increased the max to 500) until I get around 100 dwarves at which point it starts to slowly drop until eventually I'm down around 20-30 around 180 dwarves.
2)As Jimmy mentioned, DF is single-threaded, so water and creatures take a
lot of processing power. Magma will take even more then that due to also processing temperature, to the point where we have even made a
special version of the pumpstack designed to handle magma with less FPS loss. Also note that water/magma tiles that are 7/7
do not take any real power. As such the first time you connect a reservoir to a river you will suffer some large FPS drops, but once the reservoir is full and the river rises back to it's original height then most of your FPS will return. This is also the reason why the best liquid systems are either closed loop or dump back into an endless source like a river or aquifer. Simply dumping water into the caverns to dispose of it works, but is very FPS intensive.
3)While items produce clutter, the largest FPS drops are liquids, temperatures, and creatures. A note, but destroying items will not completely restore any lost FPS due to items until DF is restarted. This is because DF keeps a few pointers around for destroyed items until you restart DF, at which point they are cleaned away completely.
Lowering your G_FPS_CAP in init.txt will boost your FPS, at least in my experience. I keep mine at about 8-10, down from the default 50.
Keep in mind that your GFPS also determines how well the game responds to commands. Lowering your GFPS too much will result in the game becoming unresponsive. Also macros will break more easily (due to missed commands) the lower your GFPS is.
MOD: wrong forum, moderator, could you please move it to 'gameplay questions'?
Final note here, but due to the fact that we only have two admins (Toady, who we would much prefer to work on the game more; and ThreeToe, his brother who isn't very active) you can move or lock your own threads. Just scroll all of the way down to the bottom of the page on a thread you created and you should see an option there (IIRC it is in the bottom right).