bsd.prefs.mk vs bsd.fast.prefs.mk?

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

The pkgsrc guide mentions bsd.prefs.mk:

   The pkgsrc user can configure pkgsrc by overriding several
   variables in the file pointed to by MAKECONF, which is mk.conf by
   default. When you want to use those variables in the preprocessor
   directives of make(1) (for example .if or .for), you need to
   include the file ../../mk/bsd.prefs.mk before, which in turn loads
   the user preferences.

It does not mention bsd.fast.prefs.mk.

But it looks like bsd.fast.prefs.mk is a drop-in replacement for
bsd.prefs.mk which just serves to speed things up, at least if the
makefile ends up including it twice -- which I suspect is almost all
of them, via, e.g., mk/compiler/gcc.mk.

What is the intent behind bsd.fast.prefs.mk?

1. Should it be used by package Makefiles?
2. Should it be used by supporting foo.mk or Makefile.common files?
3. Should we mass change one to the other?
4. Should we rename: bsd.prefs.mk -> bsd.slow.mk, and
   bsd.fast.prefs.mk -> bsd.prefs.mk, and make everything just use
   bsd.prefs.mk with no change to the guide?

Whatever the answers, it should be documented in the guide how to use
these.