I decided to put a couple of real boot times rather than just say "slow". So I booted my older MacBook Pro, the Early 2013 version, and from the moment the Apple appears on the screen, a little bit into the boot process, until the useable desktop is up and ready was between 53 and 56 seconds. (Although, very interesting, one boot where I was not yet timing how long it took, it clearly went FASTER, hmmmm). So that machine is running High Sierra 10.13.2 with FileVault ON. And the above mentioned times included how long it took me to enter the rather short password on that machine.
Contrast that with my newer, 2016 MacBook Pro Touch Bar which is still running Sierra 10.12.6, also with FileVault ON and again, including the amount of time it took me to enter a harder password (harder than on the older machine above), which took right around 24 seconds to boot.
So twice as long plus a few seconds.
I do know how to clone and I do so routinely on this new machine using SuperDuper. I clone often. In fact, when the older machine above was the "newer" machine, I did something once (long story) that absolutely killed the machine. I did something risky related to going down deep in a Library file related to VMWare Fusion and running Windows on that machine. I booted first from the clone and did the risky change on that clone and it worked perfectly. I then did it on the main drive, carefully and precisely as I had just done on the clone and killed the machine dead. Every character, everywhere on the machine, showed up as a "?" with a circle around it... oh my... But I reversed cloned for the first time ever has I had always been to chicken to try it before and ta da, it worked perfectly. I also reverse cloned that same, older machine above more recently when I was shocked to see how slow High Sierra was on booting so I reverse cloned back to Sierra (what I had on purpose on the clone) and the boot load times went right back down to where they had been before. Then I have since gone back up to High Sierra after Apple put out on more update to 10.13.2 to see if the boot times would get better but no. Still pretty slow.
I suppose some might tell me that 53-56 seconds is ok and just live with it. Or stop turning the machine off at night. But I'm just sort of surprised that the world isn't whining about slow boot times. A few are, but it's not a huge wave of complaints. And nor on either of these machines to I have what I would call any real risky software other than maybe running VMWare Fusion and Windows on that older machine?? I only have Windows on that older machine and other than that being the only place from which I can currently run Windows, that machine is otherwise totally my test machine. I run new OS's on that machine before putting them on my newer TouchBar MacBook Pro.
Regarding shutting off TRIM, I will likely not do that for the reasons you suggested. Even if that showed me some promise on that older machine, I doubt that I would want to try that on my 2016 machine.
I will likely just punt soon and accept that not all "progress' is in the forward direction. sigh...
thanks for all the input.
I still am going to do a boot on that older machine where I am going to try to time exactly when that progress bar stalls and sits for the longest time. And then if I can come close, within seconds, of what actual time the stall starts, I am going to go stare at the System Log and see if I can see anything that might suggest some process say getting into some loop or whatever??? I know, I've watched the System Log before and I am not close to smart enough to know what I'm really seeing. Some of you are UNIX experts. I, sadly, am not.
And I might also try a SAFE boot soon as I see others suggesting here. I will play a bit more but will soon want to go ahead and bring my new machine up to High Sierra as one can't stay on the older OS's for long, especially with the need for staying on top of the newest Security Updates now that we have these new hardware bugs (Meltdown and Specter) that folks are trying to patch like crazy.
thanks for all the help... bob