But it's terrible for performance. The performance of trivial ajax requests is completely dominated by framework setup time, in my experience with Zend based code.
"Trim that fat"
The "fat" is something which is useful to a lot of people for development, it's just a performance hindrance (and far moreso than in other platforms).
That said, I've had some Java folks express some amazement at how fast PHP is, even when it's doing all the library loading and class instantiation on every page request. In multiple cases, they all expressed that they thought it would be far far slower, having grown used to "object creation is slow" in Java.