Ok, so I hope I am not the only one that has noticed this, but Sieging in this game is almost unplayable. I play on Ultra unit sizes, because why wouldn't you. I can max out this game fairly decently and run large scale battles with functioning FPS (fluctiative though). However, sieges is another pickle entirely.
As soon as the units decide to scale the walls, it becomes unbelievably choppy. Freeze lags and stutter beyond your wildest imagination. Combat on the walls is just crazy, you can't command your units properly. Reminds me of sieges in Shogun 2 where you at one point you cannot move your units. They freeze in position, like the game cannot find a path for them. Both the player and the AI suffer from this.
The performance fluctuates in a manner I have never seen in a total war game. It can suddenly rum 100% fine, then it all crumbles and all freeze and stutters. Only in siege battles does this phenomenon happen, as far as I am aware.
Do NOT play realism mode if you intend to siege. You cannot pause and make moves or slow down. You will deal with immense lag.
CA, you have to get into this. I am sure everyone suffers from this exactly siege problem.