There is very little difference between the performance of a PHP run site and an ASP.NET site.
Both ASP.NET and PHP web applications are extremely scalable languages. What matters more to scalability than the language you choose is the development talent you hire
Because PHP is open source its pool of developers is far larger than ASP.NET (which is windows based). That being said both boast vibrant communities that post regularly to online forums, so if you’re looking for answers to problems, you’re likely to find both communities helpful.
While you’ll most likely be able to find answers to your questions in both communities, you’ll almost assuredly get those answers back more quickly when working within PHP forums.
PHP is an open source, and thus, completely free, while ASP.NET is owned by Microsoft.