There are many threads involving this fix, but I wanted to make a nice thread with easy to find info.
And the proper directions on how to do it.

I do not take credit for any of this. Just making helpful info easier to find.

FIRST: To fix your FO:NV lag issues involving npcs. This has been confirmed by numerous people
Visit this link: http://segmentnext.com/2010/10/19/fallout-new-vegas-errors-crashes-freezes-fixes/
Follow tip # 14's directions.

The page in the above link also includes many other helpful fixes for many problems for FO:NV!

SECOND: Make sure your DirectX is up to date.
Visit this link: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=2da43d38-db71-4c1b-bc6a-9b6652cd92a3&DisplayLang=en
This small installer will update your DirectX with the latest components.

THIRD: Make sure your graphics drivers are up to date:
Latest Nvidia drivers are version 260.89 - follow this link: http://www.nvidia.com/Download/index.aspx?lang=en-us
Select your correct system info and download.

Latest ATI Catalyst version 10.9 - follow this link: http://sites.amd.com/us/game/downloads/Pages/downloads.aspx
Download the correct drivers for your specific Operating System.

I will update this thread with more helpful info and fixes in the near future. (I am currently at work )
This is to help people experience FO:NV the way the developers had intended, at least until they release proper fixes for all these bugs!