I think what you describe is called tech inflation and is indeed there. It can get pretty strong on some techs.
Say for example that at the begining you could research "planetary invasion" in e.g. 20 turns.
If you now first research a couple of other techs (especially weapons I think, but not sure here) the costs start to increase after a while. So it can happen that even if you have more research points now, you take longer to research "planetary invasion", say 40 turns.
IMO it's a somewhat strange feature.
My theory is that this explains some strange AI behaviour I've seen, too:
In some games I have AI's that colonize half the galaxy, but never ever research weapon techs.
In the end they either surrender because of a total lack of military, or they finaly manage to research weapons and within a couple of turns crank out enough ships to totaly dwarf everyone else with their military rating.