How long it takes to build an improvement is related to how many factories you have on the planet, your racial social bonuses & econ SBs, and of course what you can afford. There's no point in having half a dozen factories on every planet if you can only afford to run them at half their capacity.
1) Build more factories.
2) Build more stock markets (better economy).
Edit: Oops, forgot something.
Also, if you have a planet which is of a type that you don't have both levels of tech for (aquatic, heavy gravity, radioactive, toxic, barren), your production output on that planet will be reduced, to 50% if you have only one level, or to 0% if you have neither (for instance, if you acquired it via invasion).
