This is my experience as well. The early game runs well enough but once you get the world reveal and have my players visible it's a turn waiting CPU chugfest.
TechSpot appears to have a history of doing CPU and GPU scaling tests with main new games. The recommend a quad core and a desktop HD 5750 or GTS 450 for an enjoyable experience.
It is an all around hardware consuming game. Surprisingly enough for a turn based strategy one. My friend running on a Core 2 Duo with Crossfire 4800s wants to upgrade just to run the game more smoothly.
This is probably due to the fact that it handles/renders many thousand little units at the same time, even though gfx don't look that astounding in their own right. Starcraft II faces the same situation...but seriously, a quad-core required to play a game? Ridiculous.