All points that are discussed here are right (in the point of view). But again .. if you reach such a dimension of traffic i think you have a complete other workflow, a bigger team and other problems ;o) To start an idea, a site or a platformm, CI is for my opinion the best and fastest way. If needed you can refactor the site. The money you save on the rapid development with CI can reinvest later on.
HHVM .. never tried them but i sound interesting.
