I can run Crysis with everything on max detail while running folding@home in the background (in CPU mode) and still run Crysis at over 140fps.
While you may say to yourself "more expensive does not mean it is strong" to use as a defense, I once again stress 140 dwarves on a 10x10 (which I might mention I pointed out first because of the fact that I knew this argument would come up which is wrong) at 70+fps.
This machine is not weak. I am running a powerhouse of a machine that can handle bloody well anything you can throw at it without a hitch, CPU or GPU is irrelevant. This computer is no slouch. This is why I mentioned that 80 dwarves more on a larger map 5-6 years in I still get 70fps (give or take 5), as compared to on a small map just by making water or magma flow - which dumps to 22fps.
Trust me here. My computer is strong. I build it for strength, not for cost. I did not say "I have a really expensive computer". I said "I have a beast of a machine". These are two entirely different statements.
Well, you probably have a quad-core then, and something akin to a SLI GPU setup, plus a load of RAM. Out of all that, DF benefits only from the RAM, and one of the cores. There are no graphics to speak of, and Toady didn't have the skill to start making DF with multicore CPUs in mind, so your "beast of a machine" is no better for running DF than a medium-level office machine with a 3GHz dualcore, four gigs of RAM, and a crappy videocard. Indeed, my laptop may be more efficient at it if I double the RAM in it (and if it actually worked, but that's beside the point).
Hm, somehow I started this reply without wanting to start an argument. Well, anyway. DF is vastly different from any commercial game in its requirements.
Especially from Crysis. And the fact that flowing water and magma bring DF to a crawl on a small map means that DF doesn't like your PC for some reason. My feeble old desktop manages to keep working at acceptable speeds when flows are involved. I don't remember the exact number, but embarking on a narrow waterfall in a 4x4 area gave me around 30 fps. I gotta check that again btw, it's been long since I ran DF on this rig. Maybe I'm even wrong.
edit: yes, wrong, and in a good way. Just embarked on a 4x4 site with a crossing of three minor rivers, meaning two waterfalls. The large space between the rivers constantly fills up with water, I'm digging channels around to let even more water through (creating Niagara falls, as it were), and I'm still constantly above 85 fps. This is on a rig so outdated it ain't funny - a 2GHz P4, with 512mb RAM and a GF4MX440. Which means that if you suffer terrible lag with flows, something is wrong between DF and your computer.