Main reason is that this two threads load most of later required classes which can later be quickly loaded from multiple parallel threads.

Currently concurrency causes that 8 -16 threads (on 4-8 logical core systems) try to load same classes at same time.

this leads to lots of contention as result. "preloading" some of this classes reduces contention.

Looking at the list in the current "hack impl" there are lots of classes that don't need to be there, stuff like subsystem parsers which are only loaded once in any case.

Main pressure is on classes from jboss-modules, controller, server & xml parsers modules, all others are not as problematic.

This is also reason why lots of contention is happening on JDK classes as well as those are shared between all parts of server code.