You trade having to compile for actually having code that can scale

Not sure what you’re talking about, I scaled to millions of users on a pair of boxes with PHP, and its page generation time absolutely crushed Rails/Django times. Apache with mod PHP auto scales wonderfully.

It scales just fine the same way everything else scales: put a load balancer in front of multiple instances of your app.

It can scale by the virtue of spending a lot less time processing the request

You don't know anything about the PHP ecosystem and it shows.