If the bike fits you, and it's appropriate for the length of ride and course conditions, then it's only you that is going to make it fast. So what ever is in your price range. When you get yourself maxed out in performance, then you can figure out where some improvements such as aerodynamic, time trial bike vs road race, endurance or cyclocross might be better.
If you are going to be doing competitions a lot, then consider getting a coach. They'll have a more balanced idea where the big gains can be made for you.