This will give you a LARGE boost in performance (Or rather, it should.). Especially in high NPC areas. I'm sorry if this is in the wrong section, I just wanted to help out as it took me a bit to discover these but it was very helpful.

In the ~ (command) menu, type "toggleemotions". NPC's will not display facial emotions any longer, and will give you a bit of a framerate boost.

Ontop of this, if you're looking for a large boost all across the board and the cost of graphics, open up your Fallout.ini AND Falloutprefs.ini and find this line:

bDoSpecularPass=1

Go ahead and change that to 0 in BOTH ini's. It will not work unless you do both.

What does this do? Simple, specular mapping is what causes those reflections you see on everything (Stand near a light source and move your camera around. Your armor may reflect, and metallic objects / streets will reflect.). This will make your game WAY uglier but should give a large FPS boost.

Hope this helped! Sorry to wall of text you.

Another thing you could do is open the console and type "tlb". It makes everything lit up. It makes the game very ugly and ruins the atmosphere, but it gives you a HUGE FPS boost. It also works for Oblivion. The command is in Skyrim, but it sadly doesn't do anything.