The startup time can be pretty extreme -- although you can mitigate much of this with various caching strategies but the typical PHP request involves reading dozens to maybe hundreds of files from disk, compiling them into byte code, and then executing that byte code, and then deallocating everything.

And even if you cache everything, you still have the code necessary to setup the runtime environment of you application which runs on every request and does the same thing.

But still, I do actually like that model and it does have many advantages for development. But it's really hard to work in PHP, the language, after using something better.