Not completely true, PHP is a fast and powerful language capable of handling almost any reasonable web task. Almost everything is made with PHP, the forum we are talking on is made in PHP, other massive sites like gamefaqs.com and many others are made with PHP.

It is true that ASP.NET is precompiled, therefore faster. PHP is interpreted then compiled. However that wont make a notable difference on the project like this. Compared to forums and CMS's, this is relatively small.