http://www.tweakguides.com/
just so you know..
Thanks for posting that. I did some investigating on that .dll a few days ago in this topic: http://www.gamesas.com/index.php?/topic/1130068-d3d9dll-some-things-dont-look-right
Koroush is correct. If you have an ATI card and used the original .dll file then it would have used nvidia shaders which are not entirely compatible with ATI/AMD cards. Since then somebody modified the .dll to spoof an ATI card to load the 'correct' shaders, but I have also read many ATI users say they do not suffer the NPC talking lag so it might not be needed
Basically, the only thing the .dll does is spoof a different video card. When the game loads, it sends a command to the real d3d9.dll to find out the name of the card and from there determines what shaders/video files to load. A file named rendererinfo.txt is then created in your documents\mygames\falloutnv folder containing this information. This .dll runs alongside the real one so when the command is sent, it will respond with a different video card then what you have. In the original .dll, it will spoof an nvidia geforce 7900gs. This has since been modified to be an 8800gts but IT DOESN'T MAKE A DIFFERENCE. A bunch of morons here and elsewhere have been saying "there is a new file out that uses an updated graphics card so you get better facial animation and shaders". Complete and utter bull crap. The rendererinfo.txt files are identical between the two. The exception would be if you have an ATI card and get the ATI version.
Although I initially thought something looked off, I made screenshots and videos in FRAPS of multiple scenes in daylight, night, interior, exterior, heavy HDR, water, and faces. If you use the .dll and use the right one for your video card manufacturer, the good news is that the everything is identical - HDR, lighting, water, etc. Except for the facial animation.... It's more plain and stiff NO MATTER WHAT DLL YOU ARE USING. It's the same as Fallout 3, but apparently in an effort to make the characters less robotic they made subtle changes to the facial animation. Without the .dll you can disable emoions ni the console (like it says on tweakguides) to achieve the exact same thing. But you won't get achievements if you use the console unfortunately. These animations are so subtle though, I don't know why there is an even a performance hit. It's things like eyebrow movement and eyes squinting. When a NPC is talking, my CPU and GPU are vastly under utilized. Hopefully this will be fixed in a patch. (There is also slowdown when fighting somebody that attacks with melee weapons, probably do to the new melee animation)
The only difference between using the .dll and not using it is the driver file that is called. In renderinfo with the .dll it tries to load nv4_disp.dll. This is nvidia's older display driver file and doesn't even exist on my system. Without the .dll it loads the main direct x driver file - nv3dum.dll and for some reason that causes slowdown
For all the people on here that say "it's because of direct x 10/11 and this forces it to run in dirext x 9" - you're absolutely wrong. Stop spreading wrong information and next time investigate what is going on. It's nice to know that the slowdown goes away, but don't you care WHY it goes away instead of blindly using a file somebody else told you to?