My impression is that cperciva has been working on FreeBSD/EC2 boot times for the last six years, not just specifically some changes in 14.
The most accessible summary of his work is probably on his Patreon: https://www.patreon.com/c/cperciva/posts. If you go digging there are also some anecdotes on his Twitter too; most recent I can find is https://x.com/cperciva/status/1833735559614988526