In many cases macro-optimization trumps micro-optimization. C/C++ might be closer to the metal and in extremely simple applications incrementally (or perhaps significantly) faster, but if the difficulty of using C for web app development results in an extremely sub-optimal solution then you'd have been better off with a more advanced language like Ruby or even Java.
Moreover, the long pole in almost every web application of any significance today is the database. Whether you're using C++ or PHP is irrelevant if just waiting on a SQL statement to execute dominates the performance of your app by an order of magnitude or more.
Additionally, there's frequently a lot of room for optimization in any web app. Techniques like CSS spriting, serving static content from a cookie-less domain, and aggressive caching can make far more of a difference to overall performance than choice of language.
Finally, throwing hardware at the problem almost always makes sense as long as you can get away with it. A $1500 server is roughly equivalent to the cost of a single day of development for a talented team of only 4 devs (keep in mind that the cost of employing developers includes not just their base salary but also other costs like administration, 401k matching, health insurance, payroll taxes, etc, which nearly doubles the nominal per hour cost). Until you're perhaps $20-50k in the hole it probably doesn't make sense to consider putting serious effort into re-engineering your software from the ground up for scalability.