Well, I am just going to throw some shit in here. I am not even sure it's going to help.
Don't hate if it's bullshit.
De fragmentate your hard drive (It's handy anyway)
See if there was a graphics card update.
Delete all mods for M&B Warband, except NW.
When playing, close all programs that use excessive bandwith or internet:
Skype, Spotify, Utorrent, like that.
Hope this helps.