Honest question: while I know that's true (I was using APC, now using Zend Opcode or whatever the replacement in PHP 5.5 is whose name escapes me at the moment), it seems to me that there's still going to be a performance penalty incurred. The PHP may be all compiled to bytecode at that point and the bytecode may be 100% memory resident, but you're running through all the initialization routines for the framework on each and every page hit.

"Pure" PHP is a pretty fast language when benchmarked, but PHP frameworks tend to benchmark poorly, and I've always assumed that the overhead incurred by PHP's execution model is the culprit -- essentially, PHP was written with assumptions about How Dynamic Web Sites Work that made sense in the late '90s but really suck when every request is hitting a front controller and being dispatched through a router. Is this not the case?

Yes you can use APC, opcache or XCache to load php bytecode in runtime. but you are still calling php functions/methods or creating a lot of objects in runtime. that's the overhead.