I'm quite convinced the higher the difficulty level, the faster you can finish your science victory. Same goes with culture as well. The thing is AI needs to have things that you can take - be it through trade, espionage or war. On higher difficulties AI have more stuff earlier and that helps you. A possible exception is if you go completely pure-hearted and pacifistic and don't even use spies offensively - then maybe lower diff is faster. Domination and religion are faster on low difficlulty; science and culture it's the other way round. Although it could be argued you can build 100% culture and ignore everything else if you played on chieftain or something, perhaps win quickly - but I bet you'd win more quickly still if you used two or three other civs' culture products in addition to your own... Plus AI having money to give to you is invaluable (which makes deity trade much more effective than lower levels trade).