fpsPlus and optifine are mutually exclusive, they both have the option to change the math functions to a faster one, so if you have optifine you dont need fpsPlus
I could be wrong, but I think that (at least in my case), FpsPlus's fastmath gives better performance than Optifine's. With all 3 of them (along with the other 100 or so mods) I went from around 27 FPS to 60-100+ (depending on what I'm looking at).
In any case, thanks for making me realise that it's not FastCraft but FpsPlus that was the cause.