That game is a system hog. Its fine on the large world map but as soon as you enter battle scenes it really slows up as there are thousands of men on the field.
Its a combination of factors playing it on the lap top. A new graphics card or memory will help but will only identify the next limiting factor of playing it in a lap top, which I guess may be the CPU.
Have you a desktop you can play it on?
Finally, have you tried running the game at lower graphics options? Also look to close out all the background tasks that may be using the CPU and memory.