Currently, battles take ages
If you engage a stack of 9 with a stack of 30, the battle takes super long, even though you have 3 times the numbers.

Lets give an example
Say you are Austria fighting France, you would be split between your Austrian and Lowlands front, and AI France would probably split up his troops.
If you keep your stack together (because you have less troops than France) to try and take out smaller stacks, this is impossible due to the completely unrealistic long battle times.
If you engage a French siege force of say, 10k in Luxembourg, with your entire army of 40k, the French and co have enough time to move ALL their troops all the way from Southern France back to Luxembourg and reinforce the battle, which means that you will always lose battles.