I've been playing Command and Conquer 3 with the Tiberian Essence mod, but I know that's not the problem, because it happens on vanilla TW too.
However, Tiberian Essence comes bundled with new maps (Frozen River, Dueling Islands, Dueling Islands Classic, and Tiberian Wasteland).
On the Official EA Maps (like Small Town USA, Barstow Badlands, Tournament Arena, etc.) the games periodically slows down, until it becomes hardly playable. However, on the Tiberian Essence Maps, the game runs smoothly, and never slows down.
It seems that something is wrong the the EA Maps that causes my computer to slowdown. If anyone has any insight it'd be greatly appreciated.
By the way, here are my specs (which should be easily able to play this on ultra high)
intel Core i7 360QM (Quad core with hyper threading, essentially 8 functioning cores)
nVidia GeFORCE 630M 1GB
Windows 7 64-bit OS
8 GB of RAM