I am not professional, but here is my answer: I think that slow boot has something to do with hardware and not software, although "heavy Kali" can be slow except if you bought 5000 eur PC.
so, slow boot can be related to your HDD write/read speed (22MB/s and 8MB/s), your processor, RAM, etc.
maybe you use Pentium 4 from 1998
I installed Kali on USB device and it was also slow, just not so much slow like in your case, I think it is related to above mentioned things + "heavy kali with million software", although I use computers with 8GB RAM and i3 CPU 3.6GHz.
I never read if there is "minimum required hardware" to install Kali.
