If you have Win 10 fast startup (fast boot) enabled, times can vary on what it has to do during boot. I have an NVMe and have fast startup disabled as its fast without it.  Fast startup disabled and it should boot the same each time.  
I have a Latitude 7490 in which I installed a Samsung 970 EVO Plus NVMe drive.  It is blazingly fast, except on startup/booting.  Sometimes the PC boots in five seconds, other times, it takes much longer.  I have reset the Real Time Clock (RTC) as I was getting the Dell boot logo twice.  Just curious why the boot times vary.  Thanks in advance. Steve
Latitude 7490 with Samsung 970 EVO Plus NVMe SSD; Why Do Boot Times Vary?