Section access will always need some overhead to evaluate the authorization and maybe perform a data-reduction. I have never tested the opening times with and without section access respectively with different granularities of section access but I think there will be rather not much potential to optimize.
Another point which is very important for the opening times is if you have enabled data-compression (in document properties in tab general in top left area). If yes disable it - then it saved only storage space but cost time and ressources by de-/compressing.
Beside them you could consider to optimize the datamodel itself and the objects within the gui especially those one on the landing page (which might need some conditional restrictions).
