It's not -entirely- irrelevant.
As the number of cores increases, the individual speed of each component core does tend to decrease as well. My old Pentium 4 would have ran DF faster than my Core 2 Duo.
In essence, from a DF perspective, adding more cores is paying good money to decrease performance.
And that was only relevant as of the first generation of chips, as of now most your duals and quads are well past any single core the average person can buy and most the quad cores are at par if not faster then the first gen dual cores on a core to core comparison.
Lowend last generation intel quad core, Q6600, runs at 2.6 per core. New generation and currently same priced dual core E8500 runs at 3.16 per core.
And that quad core is as fast as most the dualcores of the same generation, only just recently falling into the low price bracket.
But this is neither here nor there. yes DF would run like crap on really most portable devices that aren't a full fledged laptop or have something equilvent to a @ghz or higher per core speed.
EDIT: though to your main point actually, even with a slower clock speed per core, the first gen core 2 duos could out process, per core, the P4 dualcores or even the single cores, unless you were compairing some high end single chip vs a low end core 2.
clock for clock core 2s are much better.. usually about a 1.5 or 2 to 1 ratio in work that could be done per Mhz compaired with the old stuff. and that trend continues as bus speeds continue to pick up with each new generation.
assuming toady will keep working on the game for years to come.. and with what he wants in it i'm sure he will.. then it won't be long till we have hand held machines which equal the processing power of todays PCs. the atom and other micro cpus that just came out show this well.
EDIT2: sorry if that turned into a rant O.o; please by all means continue the topic around the hole i dug for myself.