This is a really good summary of some of the main reasons - more interesting detail than many of the other comments here. Now what would really be interesting

This is a really good summary of some of the main reasons - more interesting detail than many of the other comments here. Now what would really be interesting is if we had the same kind of knowledge about the typical causes of userspace software and distributed systems being slow to start. In my experience, for proprietary software, it's mostly because people inserted random calls to sleep()...