Well, as a topic name states new geforce drivers do lower perfomance in warband on some GPUs.
Personally I have a rather old PC so that it can run warband on 120 fps with no problem, but in Warband with better graphics and higher vegetation I have to run it on 70 fps cap with all but fancy water reflections and grass enabled:
I have intel dual core 3Ghz processor, 4 Gbs of ram and NVIDIA Geforce 250 GTS running 64 bit windows 7.
After not playing for a while I noticed strange lags during battles that I never had before and while ping was usual (yes I know current server host has issues with ping, but Im describing different issue) I started to suspect performance, thing is you couldn't notice it clearly on some maps. So I did a little bit of testing on most lagging map on my PC - Faernebo (swamp with a house in the middle) and was amased on how low my framerate lowered there it jumped from 70 to 50 all the time (with 70 fps cap). Further testing revealed the problem:
Test map Faernebo:
NVIDIA GeForce 250GTS drivers for 64 bit windows 301.42(Latest stable ones I had) - 117-120 fps (with 120 fps cap)
NVIDIA GeForce 250GTS drivers for 64 bit windows 306.23 - 56-75(!) fps (with 120 fps cap)
NVIDIA GeForce 250GTS drivers for 64 bit windows 306.97 - 56-75(!) fps (with 120 fps cap)
Twice worse performance! TWICE!
So there you go - if you wonder why it started to lag tremendously it very well be new nvidia drivers, especially because warband tends to lag very hard if fps drop below the fps cap you have set. Thing is I don't have any problems in other games or applications with those old 301.42 drivers so its a solution for me for now.