Along with the game level, you have to choose “Game Speed”. Game Speed determines how many turns it takes to construct units, buildings and wonders, how many it takes to research technologies, build improvements and so forth. These settings let you experience the full epic sweep of time inherent in Civilization IV while allowing you to customize the game based on how much time you have available and other preferences. There are three game speeds to choose from:
Of course, Game Speed does not have any effect upon the amount of time you have to complete a turn – you always have as much time as you want in a single-player game.