make release: how utilise filemon?

hello,

building a release of most recent 12-CURRENT seems to be at some point very
annoying. When building the main host's system out of /usr/src
using /etc/src-env.conf set with WITH_META_MODE=yes, build time decreases
significantly. Now I perform some tasks building release. The source tree is
11-STABLE.

In release.conf, I tried setting

## Set to use world- and kernel-specific make(1) flags.
WORLD_FLAGS="-DNO_CLEAN -j $(sysctl -n hw.ncpu)"
KERNEL_FLAGS="-DNO_CLEAN -j $(( $(( $(sysctl -n hw.ncpu) + 1 )) / 2 ))"

without any noticable effect - make release always build release fully, no
matter wether there has been new sources checked out or not. It takes a lot of
time compiling llvm/clang. Using 11-stable on a NanoBSD installation,
-DNO_CLEAN seems to take effect on the very same source revision, while it
doesn't on "make release". The target (obj-directory) is always properly set,
the same and not deleted by a run of "make release", so I'm quite sure this
portion of the setting is correct.

release(7) does have some tags setting src.conf, but I miss src-env.conf
settings as this can be customised in the "ordinary" /usr/src world.

What am I doing or thinking wrong in this matter?

Kind regards,

Oliver