I am playing as Portugal, I am attacking Tunis, the last holdout of the Barbary Pirate scum, and the lag is so terrible that I can't even play the battle. They have a Star Fort. After my 12 pounders fired at their wall for awhile the game just became so slow that I couldn't do anything. 4X speed did nothing at all really.
DON'T DO IT! Maybe if your computer is great you can, but mine is not.
Maybe if I try to storm the walls it will be different, but I don't think I should risk it.
When the battle is a sally battle that is different. It does not seem to cause the same lag. I wish I had not enabled forts. I might have to either start over without Forts or just siege every single fort out when I cannot autoresolve. Very disappointing, but I know it is a vanilla bug and I'm bet there isn't a thing I can do.