Fort battles FPS slowdown.

I'm playing NTW unpatched, and using the Napoleon Realism mod.

I never build forts, so never have to defend one. But I often have to attack forts, with the AI defending.

I have experienced a severe FPS slowdown as the fort battle goes on. But this usually takes the form of the FPS slowing down, over a few seconds, then speeding up again, for another few seconds. This carries on, but getting gradually slower and slower, right down from my usual range FPS range of between 30 and 40, right down to as low as 1 FPS after some time.
(As I say, my normal battles, without forts, play at 30-40+ FPS).

But this does not seem to have anything to do with melees, as I bombard the front wall of the fort with my cannons for quite some time, then, hopefully blast any enemy units that come out through the hole.
Using my cavalry units to run down any enemy units that route from the fort, before eventually assaulting the hopefully much reduced defenders with my infantry.

Not heard anything about "disabling" fort battles before, as I think I've read in another topic.

Anyone got any useful comments please.
Thanks.

Moved to Technical Help.

How are you playing it unpatched? Perhaps that's your problem. Patches help this particular game run better.

Update XP to SP3.
Use MadBoris' Out of Memory Workaround stickied in Empire tech.
Update Nvidia driers to the new 258 ones.

@YD, One of two ways: Steam's DRM will do it for him next month or it's illegal.

should it not be moved to NTW area?

Napoleon, yes.

it is normal the pathfinding bug is still present, but do as ron said