I ran into this problem and was about to post here looking for a solution, but a little bit of searching found one for me in the support thread. Since this is an issue that has been driving me nutty since PoP 2.5, I thought I would post the solution in a more visible place than page 18 of a sticky that very few people will probably read. It's really a big help.
Note that this is meddling with the occult! You may break your game. Back up all files before changing them, and please tell us if you did this before reporting any possible bugs.
Problem: Extremely low FPS and extremely high lag in hilly and wooded battle scenes. I was seeing framerates as low as 3.0 FPS on settings that usually produce framerates of 30-60+ with my rig, making effective combat nearly impossible.
Solution: (Courtesy of Lord Kinlar)
First, rename or delete the following files in SceneObj folder, these are the bugged files and you won't need them.
scn_random_scene_plain_forest.sco (906 KB)
scn_random_scene_snow_forest.sco (906 KB)
scn_random_scene_steppe_forest.sco (906 KB)
Now create a copy of the following files:
scn_random_scene_plain.sco (1 KB)
scn_random_scene_snow.sco (1 KB)
scn_random_scene_steppe.sco (1 KB)
Now rename them as the first three above by adding _forest before the .sco extension. Ultimately, you should have the following files in that directory:
scn_random_scene_plain.sco (1 KB) (Original)
scn_random_scene_snow.sco (1 KB) (Original)
scn_random_scene_steppe.sco (1 KB) (Original)
scn_random_scene_plain_forest.sco (1 KB) (Copy)
scn_random_scene_snow_forest.sco (1 KB) (Copy)
scn_random_scene_steppe_forest.sco (1 KB) (Copy)
What we need is to get rid of those 906 KB files and use copies of 1 KB ones instead of them.
These files can be found in c:\Program Files\Mount&Blade\Modules\Prophesy of Pendor 3.01\SceneObj
This fix immediately produced a nice, crisp 60+ FPS for me in the same scenes that nearly ground to a halt before. Needless to say, it's a major relief.
Hope this helps some other people, because it certainly did me.
Original link: http://forums.taleworlds.com/index.php/topic,53320.msg2967971.html#msg2967971