This paper written by Colin Percival, known FreeBSD contributor, describes his work along with several others profiling the FreeBSD kernel boot. Colin goes in depth on timestamp logging, function tracing, boot holds, and provides a visual flame chart of all the devices at work. See the link below for his detailed paper.

Profiling the FreeBSD kernel boot: http://www.daemonology.net/papers/bootprofiling.pdf

More publications by Colin: http://www.daemonology.net/papers/