Show me a website where php is the source of the performance problems and I will show you a website php allowed to reach success/a large audience.
Frankly, unless you are very large scale php is never your bottlenecks, I/O is. And if php become your bottlenecks, you have already won.
(of course you could find exemples of math-heavy calculating sites where this isn't true, but that just means you don't know how to pick the right tool for the job)
You are right that I/O is usually the first bottleneck. I assure you that being CPU bound is no picnic, not least because most optimization knowledge and experience out there relates to I/O. :)
Even if you are not CPU bound, HipHop is steadily adding grown-up features to the language that make it quite nice to use, functions like mysql_connect_with_db() which save a database round-trip, and extensions like xhprof which will tell you what is actually slow.
Frankly, unless you are very large scale php is never your bottlenecks, I/O is. And if php become your bottlenecks, you have already won.
(of course you could find exemples of math-heavy calculating sites where this isn't true, but that just means you don't know how to pick the right tool for the job)