From the 17th century onward....

[Drake44444]

Up until the mid-late 17th century may game runs fine, then slows down to the point I make fewer moves and then I just would rather start a new game. How can I stop this problem?

[joncnunn]

Sounds to me like your machine is marginally insufficently powerful for the size of map you play on combined with the graphics setting.

I'd first try lowering the graphic settings when it starts to slow down to much for you.

If even low graphics is insufficent to advert too much of a slowdown, then you could try to win the game via dominiation before then. At the very least, if the computer AI controls less units, they won't take quite as long to move.

Or you could try playing the next smaller map size.

And it wouldn't hurt to see if the harddrive needs defraged.

The other free thing you can do is increase the virtual RAM to 2.5X the RAM you have.