This gets brought up a lot, there are two threads about it right now. There are a few mods that usually get recommended, the big one is Historic Speed. But I'm just putting out the word that the one you really want is called DynamicEras. Why is it the best? Because it's fully customizable, it's just a text file where you can input tech cost multipliers for each era. If you want you could just have every era 1.5x slower or whatever, or maybe you're playing Rome and you want a 2x longer ancient and classical era and then normal eras after that. It's completely up to you. And you can even change the multipliers in the middle of a game.