Oh, I have not lag in C2 even when I use 3.000 soldiers, and I play this game on my notebook which is more than 5 years old.
May be you should make small tuning for this game on your computer.
Try this way - Cossacks 2 - Battle for Europe\Run\Data\ find log.txt open it and make READ ONLY. It should little bit help with lags.
I can agree that ACFB give you few needed things as like bigger maps, auto fire etc. Nevetheless C2 had better morale, fatigue and stamina efects.
It is so fullish when soldiers in C1 or AC can crossing over map, fighting all the time and never be tired. What is more, there are thousands crowded men fighting and killing thousands which is also so stupid, because people do not want to be killed. Normally they run away, and real soldiers many times fled when their morale was broken, and this factor is better showed in C2.
Soldiers flying from broken formations looks and works better in C2. Formations movement and fight, a specially cavalry charge and melee also looks and works better in C2. However cavalry use wrong tactical formations there and many units are not historically accurate too. Stacked formations, men and horses like gosts passing through friendly formations and many other stupid things and bugs exist in both series of the games. I pointed some of these things in another thread.
I could add here that all graphics things and even units are more detailed in C2 and that could be interesting, but who cares about small details when hundreds men are fighting. Nevertheless numbers it is not a main problem, because number of men acted on battlefield never be enough in these games.
What a really matter 15.000 or 4.000 men in these games? Still it is not half a million combatants how it was in reality at Battle of Lepzig in 1813.
Therefore you can not put scale 1:1 in the game, because you can't get there even 100.000 men. Better include less men and put more formations which could form more complex battle of order. This way you could emagine that there is much more men on battlefield than it is in simple number counting.
I am afraid that any mod do not solve these problems and therefore I asked in the beginning: is it possible that somebody can develop strategy game based on Napoleonic Wars and could it be new Cossacks game or next add-on for C2 BfE?