Hi
I have purchased a new iMac pro (27" 10x cpu with Vega 64 Graphs + 64GB Ram)
I also have a MacBook Pro (13", 2016 3,3 i7 16GB Ram)
MBP boots in 17 seconds but iMac boots in 33 seconds. Both of them have macOS High Sierra 10.3.2
There are no Login items. I don't know why but maybe it is just it meant to be! I am not sure. What do you think?
Thanks
Boot with the "V" key pressed to expose the Verbose startup mode. What you see on the screen may provide some clues what's going on. Maybe the stream of text suddenly stalls and that would then give you an explanation for where in the Startup process things are stalled and/or slowing down. Do same on the MBP and compare the results.
Just an idea to help run this issue down for you and us.... thanks.

[doublepost=1515864138][/doublepost]You might try booting up with verbose mode set. Press and hold the V key down when booting up. The stream of text may provide you with some clues, especially if the stream of text slows down or stops echoing to the screen for a while. Looking at the text when this might happen could provide an explanation possibly.
BTW... My 2016 15" MBP13,3 with 256GB SSD and 16GB RAM with all 4 USB-C ports in use takes 18 seconds to boot up from the first instance of seeing the Apple logo to the login display.
My MP6,1 6-core/32GB RAM/1TB SSD and a ton of storage hanging off the machine (TB devices, USB devices, etc) takes around 90 seconds from the boot chime to the login screen. Having lots of externals will typically cause Startup to be slower than having no externals connected.