Apache has several threading models.  You used the slowest (and default on Debian).  Try looking at Debian modules "apache-mpm-worker" (a threaded implementation) or "apache-mpm-event" (a similar model to lighttpd).

"The worker MPM provides a thread implementation for Apache HTTPD. It is considerably faster than the traditional model, and is the recommended MPM."

 "The event-Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing to work to supporting threads..... "

 The pre-fork model that you tested with is the "safest" but not the fastest (ie, not a danger for non-thread safe libraries).