man make.conf states, that COPTFLAGS is used for building/compiling the kernel
(exclusively). The question arises: are kernel modules NOT kernel or are they kernel?

The problem I face is that with optimization level -O3 loader.efi gets miscompiled and a
UEFI laptop stops/reject booting. To avoid other interference, I defined COPTFLAGS
in /etc/src.conf accordingly, but leave CFLAGS?=-O3 in /etc/make.conf for compilation of
regular ports and the rest of the OS.

I can observe that with CFLAGS set, either in make.conf, or src.conf or mutual exclusive,
the CFLAGS is ALWAYS incorporated when kernel stuff like modules and even the loader.efi
is built! I consider this inconsitent, since loader.efi is definitely kernel related
stuff as well as modules.