Havent had any trouble with the spider cave, same FPS there are everywhere else. Dont know what causes your FPS drops there, maybe too high AA and AAA paired with hairy spiders? Tho i use 8x AA and AAA myself, and i got a HD4890...
The settings from the Nehrim launcher improve game performance greatly for me, but i also have a lot of crashes, reason is that the engine is still the same as in vanilla oblivion, since SureAI dont have the possibilities to change the engine itself. And it is at its limits.