Q should of answered this, since I'm rather sure this is in the by now thousands of times asked category: Brute speed of a single core. (Well, bus speed too I guess.)
That's it.
Hell, I've had plans for years now to make a DF computer using an older single core chip overclocked to, what is the term, 'balls to the walls' speed that will live in a small tank of mineral oil. Because its dwarfy and, hells yes, we need more single core speed. And a way to shed insane amounts of heat from a small localized area. MUST MAKE DWARF FORTRESS GO FASTER.
Actually, come to think of it, with this laptop I'm using now I'm rather sure I've never seen DF get over 600-700 out of the 1GB of ram...Forget more ram, localize everything to other cores, make DF use a single core that nothing else is touching. Toady did (and still does) math. Much number crunching. DF loves eating your cycles. Memory overhead honestly isn't that important as compared to brute computing power.
More specifically, I think last time I got bored enough to hit monarch was on a 1.2Ghz dual core laptop with a gig of crap ram in xp...no, I lie, lubuntu. And a slowly dying hard drive that kept throwing read/write errors as it ate itself alive, so no paging file to keep it going as long as possible. I was down to...12-15fps? on a 3x3 with judicious cleaning up with trade/smashing. I also play with a rather low gfps (without checking, I think I'm running gfps 8 on this one) which I have found helps more then any other fps saving measure. Also, never used tiles or graphics sets or whatnot, so I have no info or idea on how/if they impact your fps.