For anyone who purchased Fallout: New Vegas on the PC, it should not have escaped your attention that the game suffers from many bugs, but Bethesda forum member 'rosienski' suggested a simple fix that seems to have addressed an issue almost everyone was experiencing with performance drops around NPC's. I can vouch personally that this fixed the issue for me.
Please note that this fix does require you to download a .dll file. Obsidian, Bethesda are not responsible for any harm done to your computer. That being said, the file is confirmed clean but if you do not feel comfortable doing this, feel free to wait for an official patch.