> That make command looks a bit weird to me. Shouldn't it be just "make
> -jN". Here you have an extra -j and an -l that stops make from
> spawning new commands if the system load is higher than -l NUM, right?
Correct. That's what the default builder does, too. And if you think
about it it doesn't make sense to overload a machine in any way which is
why that setting is best.

If you would allow overloading, the average load would be too high, and
it would take long for the average to go down again. While that is
happening processors are idle, and will not be used.

The alternative is to always force spawning -jN processes.
Now what happens if nix builds 4 packages in parallel, and all of them
spawn 4 subprocesses? You'll end up having 16 processes which is not
optimal either.