That still doesn't prevent you from needing to do that on each request.
Also, with most frameworks, they're usually quite complex; how else are you going to be everything for everyone? Complexity comes at a cost, most notably in framework land this takes the form of code with very tall inheritance structures and/or tons of dependencies.
You need Foo? Ok, Foo inherits from BaseFoo, which inherits from CoreBaseFoo, which inherits from the Widget class, which itself is a BaseWidget, etc. Let's implement a few interfaces too, and now load in various dependencies... pretty soon that simple, 5-line class that you implemented now requires 100+ classes.
Things like op-code caching can greatly reduce this per-request penalty, but it doesn't change the fact that it still happens.
Except that nothing says that you need deep class hierarchies. In fact, i think heavy use of classes and class hierarchies is an anti-pattern in PHP because of its procedural per-request model.
Typically php frameworks follow a java-style model of unserializing data into objects, loading the corresponding class files on-demand, calling API's on the objects, and then reserializing. It is _much_ faster if you treat the data as a stream, cutting it up and transforming it as it passes through your code, without ever building up an object representation, and not doing any more deserialization than a simple json_decode (which is really fast). This is in fact the original PHP model, transforming a stream of annotated HTML.
Also, with most frameworks, they're usually quite complex; how else are you going to be everything for everyone? Complexity comes at a cost, most notably in framework land this takes the form of code with very tall inheritance structures and/or tons of dependencies.
You need Foo? Ok, Foo inherits from BaseFoo, which inherits from CoreBaseFoo, which inherits from the Widget class, which itself is a BaseWidget, etc. Let's implement a few interfaces too, and now load in various dependencies... pretty soon that simple, 5-line class that you implemented now requires 100+ classes.
Things like op-code caching can greatly reduce this per-request penalty, but it doesn't change the fact that it still happens.