I don’t really know when, I suspect around 4.8.0-30 or a bit earlier that kernel boot time has noticeably increased.

There is a bug/regression report on launchpad

Quick fix ? Try 4.9 mainline ppa.

Results from my systemd-analyze:

• 4.8.0-32 = 12.331s
• 4.9 = 5.041s