If your Bess is in proper working order, the flint sharp and the pan properly primed, you'll find she can go off just as quick as a percussion gun. Take the time for load development with her and try different patch, ball and powder load combinations. Some of these smoothbores will shoot fist size groups at 50 yards and it's a matter of knowing what a particular gun likes.
At Friendship, IN, heard one muzzleloader talking about using knurled balls in his gun and how it improved the accuracy. Whatever works.
Despite Hanger's criticism against the Bess, it must be realized that most soldiers did not practice marksmanship nor were trained in it until the time of the Napoleonic Wars. Up to that point, speed was everything and aiming was discouraged so as to encourage volume of fire. Even the comb on the later Brown Bess was raised on order of the King so as to discourage aimed fire. Mind you, for a brief period during the French-Indian Wars, the British soldiers (light infantry and Royal Americans) were trained to aim their muskets.
Congratulations on your purchase. I'd like to have one myself, but am saving up for a kit (I want an earlier model with the wood ramrod). You may want to check out Blackmore's book on British Military Firearms and Bailey's two books too.