That corruption is a 15bpp surfacecache being rendered with a 8bpp drawspans. You must've crossed ultra colored lighting with something. or maybe you're using ultra and theres' a bug that causes it to render with some other spans on startup.
As for fog's FPS hit, I'd think most of the hit is just going from asm spans to the C spans, unless you're comparing the difference between Ultra and Ultra Fogged.