LPC: Booting Linux in five seconds

only if the bottleneck was CPU cycles.

and most of the time it's not.

you have serialization on a lot of things

disk I/O (reading files, searching for the 50 places a config file could be before you find the one where it is, etc)

bus access (you can't query a PCI bus for multiple things at the same time)

timeouts (waiting after sending a command to see if something responds).

timeouts frequently combine with bus access as it may not be safe to do something else until you get a response from the device you just probed for (or decide that it's not really there)

and sometimes you do really have number crunching CPU tasks to do.

multi-core systems make a big difference if you really do have CPU as the limiting factor, but that's usually not the case (and IMHO software that has to do a lot of cpu work to just start up is probably in need of being fixed)

yes, when you get down into the low single-digit bootup range on a relativly slow COU (like the eepc from the talk), you do have to pay attention to the CPU load, but if you have similar systems otherwise, a fast cpu doesn't make that big a difference on a normal distro bootup

With multi-core or hyperthreading it is worth doing some things asynchronous.
But it's a subtle but important difference between doing things in parallel and doing some selected things asynchronous.

The key thing is to do the "critical path" of the boot sequential and as tightly packed as possible, while doing non-critical (to the boot) things asynchronous.