As I see the progress bar on my MacBook Pro mid-2015 move from left to right, soon after it gets past the halfway point, it starts to slow down and eventually stops temporarily, at about the time the cursor appears. Then the spinning system cursor appears for about 3-5 seconds, and the boot sequence continues normally (and finishes).
How can I troubleshoot this? My wish would be to avoid a clean install and restore from scratch.
I previously reported the same problem under High Sierra.