Before installing the game, restart your computer, or at least restart Steam. Apparently, if the steam client has been running for a while, it causes issues for games like these when installing, updating or running them. I know it sucks, but you might have to reinstall the game for this reason.

For the same reason, restart before installing updates and before starting the game. Again, you could just close and restart Steam, but just to be sure I always restart my laptop.

Don't bother changing the graphics quality. There is a slider for it in the settings but it does literally nothing.

While playing, never ever choose the fastest game speed: Reality will start to collapse if you do so and weird things will happen that are otherwise impossible (such as teleporting units, or George McClellan making mistakes). 5x speed on either campaign map or battle map is fine.

Whenever framerates start to drop significantly, pause the game for a moment or two. It somehow helps and runs better afterwards.

Don't zoom to the lowest level too often (I avoid this at all times), it's a bit heavy on the lower end computers like mine.