You're going to get a lot of very passionate responses to this.
PHP is, from a code and performance standpoint, very similar to classic ASP. ASP.NET v1 was , according to many, many benchmarks available via your favorite search engine, 3x-5x faster than classic ASP. Draw your own conclusions.
I feel that MSSQL is a superior database solution. If you're stuck with open source, at least look at Postgres. It's less popular but very powerful.
To answer your real question: performance is a function of your toolset and platform choice, but also of developer skill and project structure. I've seen far more projects that could benefit from some healthy refactoring and optimization than I have that are limited by the platform in which they are written. It is rarely worthwhile to rewrite a large application in a completely different language. Instead, I would focus on improving your existing codebase, and looking for ways to incrementally upgrade to a platform like ASP.NET.