Steve Litt <slitt@???> writes:

[...]

> Sysvinit and Epoch don't parallel start, so on these two you're right,
> they could slow down boot a lot.

sysvinit is a program and it doesn't start anything except what's
defined in /etc/inittab. This is done without waiting for anything, ie,
once one child has been forked, the next will be forked.

The sysv-rc process control system that's usually driven by sysvinit
does run scripts in parallell unless configured otherwise. This has been
the case since at least 2019:

sysvinit (2.95-6) unstable; urgency=medium

[ Dmitry Bogatov ]
* sysv-rc: assume `startpar' is installed into PATH

[...]

-- Dmitry Bogatov <KAction@???> Fri, 23 Aug 2019 16:26:28 +0000

[startpar being the program used to run init.d scripts in parallell]