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