Civilization 5 has lately been getting quite some heat lately on the Mac App Store. Civilization 5 (OSX Version) seams to have been getting performance issues most particularly crashing. And the response of the community has not been positive about it. The game went from a nearly perfect 5 star rating to a 3 star rating currently. And this for a app featured on the Mac App Store is not to acceptable.
Although I myself have not been getting these issues I have recognized that on some more low powered machines the issue occurs far more often. Which to me sounds as if it is a optimization issue. Which Civilization 5 was never great at compared to Civilization 4 or the upcoming Rome 2 Total war.
All in all Firaxes and Aspyer have some hard issues to fix in the long run.