Buildworlds are now much faster, because they can run themselves in parallel.  Invoke it using the -j option to make.  Matthew Dillon saw a 25% reduction in time when using ‘make -j 12 buildworld’ on a 4-core system.  You may need to manually update xinstall and mkdir:

        cd /usr/src/usr.bin/xinstall
        make clean; make obj; make all install
        cd /usr/src/bin/mkdir
        make clean; make obj; make all install

It’ll also use more memory than a non-parallel build, but heck, that’s cheap these days.