hardware question...

Hello! This isn''t as much a problem as it is a question... i''ve been playing around with the mipmap lod bias in d3d, and noticed that changing it had a SEVERE impact on the framerate. In my engine i got double the fps when i went from the smallest lod (cripsiest textures) to the largest lod (blurry textures). Why does it take the hardware (i''ve got a geforce2 mx by the way) longer to render larger textures? Shouldn''t it be all the same since all of the textures are in the card''s local memory? (i haven''t filled it up, so there''s no texture swapping from system memory). Is there anyway to get past the impact on the fillrate (because the game looks much better with crisp textures)? Thanks, Simon