Druapl time to first byte

We require our drupal site to have a much quicker time to fist byte. I have researched various forums etc and we believe our website to be optimized as much as possible e.g. using APC cache . where it really fails in speed tests is time to first byte. Please only respond if you have experience in such projects. If you can show an example of any drupal websites you have produced that show better than an F in this site speed test [login to view URL]