It's caused by the latest audio driver update.  Remove it and reboot and i bet it's back to normal.  It's a recent audio driver that is causing it. I have already reported the finding to DELL support.    Happened on an entire shipment recently from them.  Ran supportassist and immediately after, the slow reboots started.  
Used to boot up in seconds (from where Dell logo appears to Windows 10 login screen appears). now it took almost 2 mins. Drivers / BIOS are up to date. I tried to disable all start-up items, didn't help. I tried to boot from safe mode, yes that helped, took few seconds only. Looked like driver issue? But don't know which caused problem.
Precision 5550 slow boot up