just as a point of science....
cast iron is not an especially good conductor of heat.
the ability to conduct heat is called "thermal conductivity"
for data & units, see: Thermal Conductivity of Metals
at the top of the scale is silver; a very close second is copper
aluminum is half those, and cast iron about 20% of 'the best'
the exact numbers vary depending on precise composition and alloys.
the ability to "hold" heat is "specific heat" - if you look up those numbers the normalized coefficients can be misleading.
the coeffcients address "heat retention" on a "per mass" basis.
aluminum, for example, can "hold" more heat than cast iron - but in the real world a cast iron pan has a lot more mass than an aluminum one.
if you construct two pans of same size and thickness, the cast iron one will weigh 2.65 times as much as the aluminum.
on a per mass basis, the cast iron will "hold" about one half the amount of heat energy as the aluminum.
however - once hot, the cast iron has a 30%+ advantage in "available BTU" to transfer to the item under cook, once further heat input is eliminated.
note that aluminum pan construction rarely approaches the same thickness of material as cast iron -
in practices, perhaps one quarter to one third the mass of a similar cast iron pan.
with the end effect that "retained heat energy" in cast iron is more like 90% to 120% more than for aluminum.
so why the historical preoccupation with copper?
copper is more dense than cast iron or aluminum, but retains less heat than cast iron or aluminum, and "conducts heat" much faster than either.
that makes copper more 'responsive' to increased / decreased heat input than either.
when you twist the dial, you experience the 'cooking result' faster with copper than when using other materials.