The number of turns is the cost (in UnitInfo.xml / the total prodction of a city.)
is infulenced by yields, specialists and buildings; mines on 'building metals' and quarries add a lot, provided that they are worked, engineers and GEs add quite a bit, but engineers cost
and so are not a good choice for a developing city, plus they require forges which adds unhealthiness as it is.
You get a production boost from a few improvements that do not require resources - the watermil (which needs a river) and the workshop (which losts one
). Mines (which need either hills or resources) are good, and they also provide a chance of finding something valuable
The best way to get
by buildings is a factory + ironworks combo, but you then need power and possibly a recycling centre to get its full potential. Nevertheless, this n a mining centre is really useful