Just wanted to pass on a little saga I went through in case someone else might be having a slow boot up in OSX High Sierra. As some might know I run a dual-boot system with OSX10.13.6 and OSX 10.10.5. The reason being I had NI's B4II and Pro53 running there that wouldn't run anywheres else. I hadn't run either in a while, finally succumbing to NI's Vintage Organs and the B3 from Arturia and Arturia's Prophet 5/VS emulations. Had some downtime over the weekend and with the nasty weather rolling in here in the US Northeast I figured I'd wipe out the OSX 10.10.5 partition.
I run a 1 TB Samsung 850 SSD on a pcie card in the slot next to the video card in my 2012 cheesegrater.
That partition was the primary with the HS partition secondary. As any Mac user knows you cant' delete the primary partition so I reformatted it to APFS thereby wiping everything that was on there. To be safe before I started all this I made up to date clones of both partitions. So once I reformatted the partition I used Carbon Copy Cloner to clone over the High Sierra partition. Took about 45 minutes.
Rebooted into the new HS partition and tried a whole bunch of stuff to make sure everything worked. PT, rX7, everything worked okay. For safety's sake so I didn't lose my machine authorizations for my Waves stuff I moved the licenses to an old dslr memory card. The reboot took a while but then using the option key to boot into other than the usual partition takes a while so I paid no attention.
Once I was happy I deleted the old HS partition leaving me with one primary partition with HS on it. I then rebooted and it took almost two minutes to get to a working desktop after the startup chime. I'm used to maybe 20 seconds for that. Ran OSX disk utilitity's repair and it couldn't unmount the drive so I restarted from the external HS clone and ran disk utility to fix things. Rebooted into the normal internal HS and still slow. Googling this I came up with the thing of zapping the NVRAM which I did using this:
https://support.apple.com/en-us/HT204063 and that did the trick.
Upshot is if you're having slow boot issues try zapping you NVRAM and see if that doesn't help you out.
PS: as a treat to myself for all that work I purchased the Shadow Hills Mastering Compressor plugin from Plugin Alliance. $99 US with code.