Sensor pixels are sensitive to the angle of light striking them. They usually are setup to only receive light within 15 degrees off vertical. The microlenses are designed to "straighten up the light".
This probably accounts for the drop off in performance with faster F ratio.
It can be an important factor. The Sony Nex 7 has an engineering fault where an ultra wide lens causes colour shift to magenta because of the above.
thanks for the post Bert. Often wondered how the chip manufacturers arranged microlenses to accept wide light cones - looks like the simple answer is that they don't.
no chip manufacturers seem to publish detailed data on cross-talk either and that might well be another problem with the slightly crazy race to tiny pixels (and maybe this effect will also be worse with fast optics).