> I think there are too many variables to offer such advice in
> release.8.

Your critique wanders into how -j isn't right.

But, actually it is right.  It's "jobs".  It doesn't say "critical
thinking about other aspects".

> Anybody who is savvy enough to build the system from source will know
> about parallel builds.  If they don't, they will probably learn about
> them relatively quickly.  At that point, they will need to experiment
> with different values to find an optimal choice for a given setup
> and context.

Exactly.  Everyone who knows about this, uses it, and makes their own
evaluation.  I think what you're saying (and which I agree with) is
that noone should use -j options until they've discovered it organically
because they want things to be faster, and they've already concluded
that they need consider the details.

It's going to sound a bit strange, but some things should not be documented.