I did some research on that file and it actually belongs to a commercial software called SwiftShader. ShiftShader's primary function is to emulate complex shader routines on the CPU, but when used in conjunction with a graphics card it is a speedier alternative to Microsoft's own d3d9.dll
Whoever has been saying that it has something to do with forcing directx 9 as opposed to 10/11 is full of [censored] because that's not what it does it all. It isn't even made by microsoft
Swiftshader can only handle up to pixel shader 2.1. This is fine for this game since AFAIK it hardly makes use of 3.0, but I think it does make some use of it hence the visual difference me and some users claim to see. You can probable achieve the same effect by swapping shaderpackage files in the shaders folder under the data directory but I'm not sure which ones are 2.0 only for which card manufacturer (different shader packages for nvidia/ati)
Update - I'm wrong
This dll was created by the lead programmer of swiftshader, but other than that it has nothing to do with swiftshader. The only thing it does is spoof a directx variable IDirect3D9::GetAdapterIdentifier into a 7900gs (or ATI with another version floating around)
That is why the size is so much smaller than the real d3d9.dll which I assume runs in parallel just fine. What I want to know is, what is the game trying to do differently on a GTX 460 (or any DX10/11 card) that causes performance to suffer? All the launcher settings are identical EXCEPT for renderinfo.txt.
HERE IS THE REASON FOR THE DROPPED FPS:
In your documents\my games\falloutnv folder is a file called rendererinfo.txt. When using the 7900gs spoof via the dll file, it will try to use nv4_disp.dll which is also what fallout 3 will try and use no matter what
I have an nvidia gtx 460 and THIS FILE DOES NOT EXIST ON MY SYSTEM, it's the main driver dll for nvidia's OLDER drivers. So without the .dll it will use nvd3dum.dll
This causes the slow down when NPCs are talking and it just might be the emotions (look at the eyes, they look different while talking)