Sounds like you are participating in a bit of premature optimization. If the application is not built, while performance concerns are good to be aware of, your primary concern should be getting the app written.
Includes are a fact of life. Don't worry about number, worry about keeping your code well organized (PEAR folder structure is a lovely thing, if you don't know what I'm talking about look at the structure of the Zend Framework class files).
Focus on getting the application written with a reasonable amount of abstraction. Group all of your DB calls into a class (or classes) so that you minimize code duplication (KISS principles and all) and when it comes time to refactor and optimize your queries they are centrally located. Also get started on some unit testing to prevent regression.
Once the application is up and running, don't ask us what is faster or better since it depends on each application what your bottleneck will be. It may turn out that even though you have lots of includes, your loops are eating up your time, or whatever. Use XDebug and profile your code once its up and running. Look for the segments of code that are eating up a disproportionate amount of time then refactor. If you focus too much now on the performance hit between include and include_once you'll end up chasing a ghost when those curl requests running in sync are eating your breakfast.
Though in the mean time, the best suggestions are look through the php.net manual and make sure if there's a built in function doing something you are trying to do, use it! PHP's C-based extensions will always be faster than any PHP code that you could write, and you'll be surprised how much of what you are trying to do is done already.
But again, I cannot stress this enough, premature optimization is BAD!!! Just get your application up off the ground with good levels of abstraction, profile it, then fix what actually is eating up your time rather than fixing what you think might eat up your time.