The "few milliseconds" was a response to "Your system will start faster because it doesn't have to do things like initialize the various pseudo-filesystems Linux has, or initialize the virtual file system, or any number of other tasks." Doing that init takes very little time and zero disk accesses. That and a few megabytes of ram are all you save by using a microkernel instead of linux in this situation.

Linux won't access the hard drive if it's unnecessary, same as the microkernel.