I'm guessing that the performance differences relate more to your server configuration and the particular software you tested. Both kinds of dynamic pages can perform well in the right environment.
It's fairly common to choose ASP for Windows servers and PHP for Linux, although one can run both on either platform.
If the PHP forum blew away the ASP forum on your server, go with it. If you like the features of the ASP forum better, then try to figure out why it's slow or try running it on a good Windows server with MSSQL, etc.
Hmmmm... I wrote my own from scratch using ASP and SQL Server. If I were doing it all again, I'd go PHP and MySQL because:
(1) richer feature set (than vanilla ASP) (2) hosting is cheaper (no/low license fees to pay) (3) independence from the Microsoft Monopoly.
I quickly found my ASP forum, which has grown to in excess of 20K lines of ASP, is far, far faster than the third party offerings, but it is frustrating trying to incorporate features that third party forums offer in ASP. Image uploads are an example where vanilla ASP fares poorly - the control with PHP is much better.
All that said, I wrote my own and so my decisions are based upon the perspective of the forum developer, rather than administrator. I even had to write all my admin stuff, which is a thankless task at the best of times...