It is the game engine that handles the AI. When you have a large amount of units, usually later in the game, the computer must handle them all. Blizzard handled this by limiting the number of total units, buildings, and critters (I am sure we all have seen the "Cannot create more units" error line) that can be onthe screen at any given time. Whereas the Westood studios have thought that the "as many as the gameplayer or computer can make" idea is better. It might be able to handle normal chracter movement, but when there comes the extra, no matter how small, things like explosions, it takes a toll on the already taxed CPU and RAM. increasing either may help, but the root of the problem is in the engine. SO until the engine becomes more efficient, don't let the game last too long, or slow downs happen...
