Not a mod, but action recommended: move again to Civ4 Technical support.
Suggested to OP: I have a computer and my games don't slow down at the five minute mark, unless...
If my civ game ever slows down to intolerable levels, I save my game. Then I reboot la computadora.
If it is still slow, this means one of two things to me: infection or correction.
I always run a fresh spysweep in safe mode, and a clean virussweep based on updates.
If there is nothing here, then correction involves: shutdown anything unnecessary including widgets, browsers, and google toolbar. If push comes to shove, I use msconfig and autoruns to pick and choose the little devils sucking up my processing power.
If your computer, after all this, isn't running like a new computer, then you'll have to resign to yourself that your video card just can't handle it. Ways to offset this are a new cooling fan for your desktop or laptop as appropriate, and often just letting your computer sit for a while works. Compressed air to clean out all the sticky dust collected. Opening a window to keep some in-room circulation going.
Other potential issues that are highly unlikely (as the graphic card, though mine is giving out so I like to complain about it) your CD drive is losing its ability to spin, your monitor has a delay because of a really long chord, a friend of yours has modified Civ assets to make it difficult to play, your brain is playing tricks on you because the game is running just as fast and you are just getting too quick to play.
