2015 MBP shutdown/boot problems - "X86PlatformPlugin Result 0" messages
I've had this problem for 2 years across 3 different Macbook Pros. I think it has something to do with the OWC Thunderbolt 3 Dock. 
If I unplug the dock during boot or shutdown, all those X86PlatformPlugin Result 0 messages go away and everything speeds up. 

[edit]

I found the problem!

I have a collection of OWC USB3-to-eSATA adapters. One (older Western Digital) drive enclosure is very fussy and apparently had a particular issue with one of the adapter-cable combinations. I swapped out the eSATA cable &amp; adapter for a different set, rebooted the enclosure and viola, everything is happy again.
My theory is that this error at boot/shutdown is indicating an external hardware fault.

The troublesome device tree in my case is Mac > TB3 Dock > USB3 Hub > eSATA Adapter > WD Drive bay 

This discovery was after I removed all the 3rd party extensions from my system on a brand new (1 day old) 2019 MBP to no avail.

It should be noted that the WD drive bay was powered down while all this trouble was happening, which suggests that there's still a keep-alive charge running for active sensing to detect plug-replug behaviour