Why it is so that some techs are done in 1 or 2 turns, while at the same time another tech, much earlier in the tech tree needs 12 turns? It seems the research cost distribution is almost random? Is it part of the design? 

The only sensible explanation, but that would mean some laziness from the GC3 team (something which is impossible as we all know) is that the tech tree having been reshuffled time and time again by expansions, they never readjusted the costs so that each tech has a cost somehow sensible and consistent with its position into the tech tree.