As an ooooold Mac user, I've learnt sometimes the hard way that even security updates can break things... (it happened to me once to destroy a friend's Firewire drive because of a known bug: that friend was editing a film and he lost everything he was working on!
— the worst and stupidest thing of it is that I had read about that bug before but forgot it when I confidently applied the &@$£§ing update...
)
So, I got used to wait a few days/weeks after an update publishing, reading if everybody was satisfied with it. Same thing for a new OS version, even more cautiously, especially since I went the hackintosh route!
Anyway, since the first OSX, nearly each new version brought new bugs while curing old ones (only Tiger and Snow Leopard were quite clean!) — actually, I've waited 10.3.9 before switching from OS9.
Also, reading how Mac users are generally disappointed at each new 10.x makes you even more cautious...
Some hints:
1) I have a secondary clean current OS partition where I make any test prior to apply them to my main OS partition; and
2) in case of switching to a new OS version, I completely clone my current main macOS partition (actually using Migration Assistant) to the new OS and try for some time to work with it and nail down any compatibility issue, etc.
Fine tuning MacOS with the help of Activity Monitor and Console seems essential to me. The hardest part is learning how to tell the normal clutter in the Console from abnormal one... But seeing the same lines or bunch of lines repeating endlessly even when you're not doing anything is an unmistakable sign.
In High Sierra, I've noticed the amazing quantity of processes running behind your back, connecting to plenty of servers for any useless reason (stock exchange, weather, etc.), many of those are useless to the average user — at least, they don't need to be running all the time!
Also, those OSes are conceived to never shut down, that's why they are so slow to become usable after a normal boot.
One more interesting thing is that they seem to fully use the power of say an i7 compared to an i3: my Darkthing #2 has a HDD but Apple Mail runs normally while the same El Capitan with the same 8 mail accounts on Darkthing #1 is crawling (even after the usual maintenance practices (deleting Envelope files, rebuilding, etc.)
As for APFS, it should be a better choice than HFS+, security wise, but it looks it's at the cost of some speed. It's been 3 years now that I use ZFS which is considered the best filesystem on earth, but the development of the Mac port is very slow: I must say that if APFS was backward compatible, I'd use it instead...
In conclusion, I can only encourage you to have a look at the Console (and Activity Monitor) and search the Internet for any strange message. Then it's a bit of work, using LaunchControl (or Lingon) to disable launch agents that you don't need — as I told you before, I never had issues disabling apsd, apsc, anything with blue(tooth), airplay, airport, cloud or wifi in its name, if you don't use them. If you do it methodically — maybe first on a test partition
— you could gain a few CPU cycles.
There are many others.
