Civilization is a voracious beast, thanks to its heavy reliance on CPU calculations. It's also packed with gigantic, randomly generated maps and hundreds of individual units, all of which must be tracked.
You wouldn't think an oversize electronic board game would need so much computing power, but tracking each player's every decision, not to mention running through complicated scenarios, quickly puts a strain on a PC's hardware. Plus, the visuals are stunning.
Sure, you can turn down your graphics setting to a blurry mess, but your PC is still churning through mountains of calculations behind the scenes. This becomes painfully obvious when you wait nearly three minutes for your machine to process a single turn.