I play huge maps on marathon. My games often go past 1000 turns. I don't think I have ever been frustrated by the time between turns. Even if it's really thinking, it's never that long. Under 30 seconds on the outside. My system is not over-clocked and while I am playing with an ati hd 5830, until recently I was playing on an NVIDIA 9600gt. Phenom IIx4, 4GB, win7 64-bit.
I don't know why you and so many others are having so many issues, but the game works well, I can assure you. I don't think it would be a graphics issue because it is taking place when most activity is not being rendered... My guess is either a hard drive issue (low data transfer rate, crowded system drive, fragmented system drive, having a very large non-ssd drive on one partition, etc), a ram issue (not having enough), or a CPU issue (not fast enough, not powerful enough, not enough cache).
It could also be related to 32-bit vs. 64-bit OS. Everyone I know is running 64-bit and Civ plays fine. Anyone else want to weigh in on this aspect? One thing is for certain -- it's NOT the game. The game plays fine for many people.