Have you tried this app called preload?
preload monitors applications that users run, and by analyzing this
data, predicts what applications users might run, and fetches those
binaries and their dependencies into memory for faster startup times.
Note that installing preload will not make your system boot faster
and that preload is a daemon that runs with root privileges.
It won't help on bootup but startup of old computers.
I have a very old Dell Vostro 1510 laptop (Dual core Intel Core2 Duo T5670) which boots Zorin OS Lite 15.3 in 45 seconds. So I'm pretty happy with that speed. And I have an SSD.
I have not measured the time exactly but W10 takes for ever to load for me compared to Zorin 15.3 Ultimate. I wouldn't say Zorin boots up particularly fast either so it must be a hardware thing, but a very much noticeable difference. I will come back to update this if I remember to take the measurement but I'm really talking about 10~20 minutes until it becomes usable and responsive, where as Zorin is maybe half that.
It's surprising because the specs are not terrible: i5 and 12GB RAM, although HDD. However on another computer (which I'm using now) it's a lot faster (I would say a good 2 full minutes) even though the hardware it's about 5 years older: i7 and 8GB.
Zorin 15.3 Lite boots up very fast even on the older machines as you described.
But when I clean installed Zorin 16 beta on them, the booting speed became crawling 
As I have been using computer since DOS 5.0 
I am quite OK with that - but not for my daughter-in-law who only knows computers after Windows XP. I hope this slow booting time will be solved in the projected release version of Zorin 16.
I usually use a booting time for Zorin 16 (1 - 2 minutes for me) to make my morning double espresso. 10~20 minutes would let me prepare American breakfast 
+ 
instead of Continental one 
Sounds like the perfect excuse to learn how to cook and eat healthier 
Wow, that is too long, in a world of SSDs my old Acer Aspire 3 take 20 seconds to boot into Zorin Os 16 Core with a hard drive.
Actually the booting time for my 9 - 10 years old Acer Aspire is much faster.
My main desktop is a so-called server grade and it has to go through much testing before it boots up. Coming from an old school, I feel more comfortable to take time to startup this over-specked monster 
The last time I was using Zorin OS, my computer boot into it in 10-seconds. This is actually common for fast computers using SSD drives. But I feel it important to mention that, if using the greatest NVME M.2 drives, like the crazy ones that supposedly do 6000MBPS read, it would probably reduce it to 5-seconds who knows lol.
But its like its already been said in this thread, its not about the speed that it takes to get you there, its about how the computer performs once your in the system that matters. Better living when your computer isn't as slow as a slug and a snail on a side walk. 
If fact, I feel the speed limiting factor exists between monitor and chair. For example, I cannot type as fast as I can think.
It depends. When you using example menu start with 5 second. Then waiting for loading all stuff it takes some 60 second or more but it is very funny watching how it loading like a hackintosh or apple operative system. I remember Mint was faster but it could change long time don't tried him.
17 seconds on a quad core i5 x230 lenovo.
About 50 seconds on my laptop with a hardrive and 30 seconds on a desktop which boots from a sd card
On my Zorin 15.3 my boot load time usually take 30 seconds or not less than 25 seconds. I don't know for the 16 version coz I'm waiting for the direct upgrade. 
Startup finished in 2.996s (firmware) + 6.009s (loader) + 2.483s (kernel) + 11.896s (userspace) = 23.385s
graphical.target reached after 11.858s in userspace
Sata 3 SSD, 6th gen Pentium, 4gb ddr4 ram
WOW, your post was surprisingly quite scientific with your attention to detail. Quite informative, good job!
Wold you mind to tell us which software you used to get those figures?
I was so impressed.
systemd-analyze
Mine is
startup finished in 5.736s (kernel) + 54.470s (userspace) = 1min 206ms
graphical.target reached after 54.383s in userspace it took 1 minute to boot. Its still decent cuz the laptop is old and slow.