Rate of discovery is inversely proportional to difficulty level. Higher difficulties will have a much slower rate of discovery.
I'm finding the opposite to be the problem on Impossible difficulty, as you might expect. It just irks me that an empire 1/4 the size of mine is making discoveries every 3 turns, where it takes me 15 turns! I even have Aristotle's Lycium! He'll probably beat me to Internet, since he seems able to build a Wonder in 3-4 turns.