I've noticed the only time Civ4 ever becomes laggy is usually by the industrial era, where you have multiples of several types of units defending every city, fleets of 5-10 ships in every port, offensive armys of 30-50+ units. With Civ5 only 1 unit is aloud on each tile, essentially 1 defender in each city, with offensive armys only as big as the land you can fit them on. Even with the obvious GFX upgrades, the memory/cpu requirment that had to keep tabs on thousands of units and what they were doing or what the ai was going to do with them every turn would be gone freeing tons of resources that I don't see what they really could do with them, unless they make some really amazing AI (like more advanced ai then any ai to ever be in a game).