If you are sure that your PC is able to run Warband smoothly on highest settings, maybe it is a case of some unoptimized mods which cannot handle large battles. You must bear in mind that not every mod has been brought to later stages of Warband development with new engine possibilities and stuff. Try WSE/ WSE2 to see if the problem still persists; 'WSE' stands for Warband Script Enhancer and has two main versions; each allows to make full use of hardware, as it enables the engine to run in all cores and generally optimizes it.