High Sierra on 2019+ intel iMacs

[mike18xx]

I checked Activity Monitor for "idling" Memory usage on my Hack at left running Mojave 10.14.6; summary is shown below. Have not compared it with High Sierra on my backup computer ("Mini-ITX 3" below). I'll do that llater.
View attachment 563315

My guess is that you would use the same method in reverse as you used before to get from APFS to HFS+. But have never tried it.

No experience here with DosDude stuff, but the Apple full installer from the link I uploaded does the job and will provide a legitimate version of 10.14.6. Do what's right for you! And Good Luck!

That's not bad, although I suspect your display is of a largely bare-bones Mojave before encrusting layers of third-party apps are installed. (I find that turning all of Apple's snoop utilities e.g., Siri, problem-reporting, feedback, etc., and turning off Notifications, Spotlight Indexing, and a few other things with Terminal commands results in a healthy speed boost. Also trash any google, adobe, or microsoft thingies found in Library/LauchDaemons. I've managed to get High Sierra down to arround 2.3gb at-rest usage on 6gb systems, and it'll compress a bit more on 4gb systems before resorting to Swap. (It's still got some unfound pigs under the hood, though, as 2gb at-rest is still a lot more than El Capitan at-rest usage, as that OS was designed to be operable on 2gb machines.)

What to see substantial speed gains? Create a Fusion merger of an internal PCI "blade" flash-SSD + big 7200rpm rotational, format it to HFS+, and clone your current Mojave into that. A full reboot cycle then takes only twenty seconds from hitting the confirmation to restart, to having a returned desktop. CS6 Photoshop Extended launches in two seconds. (Meanwhile, opening any 64bit-era app on an APFS-formatted volume takes an absolute eternity, even if run from a pure SSD.)

 

[wstrohm]

What to see substantial speed gains? Create a Fusion merger of an internal PCI "blade" flash-SSD + big 7200rpm rotational, format it to HFS+, and clone your current Mojave into that. A full reboot cycle then takes only twenty seconds from hitting the confirmation to restart, to having a returned desktop. CS6 Photoshop Extended launches in two seconds. (Meanwhile, opening any 64bit-era app on an APFS-formatted volume takes an absolute eternity, even if run from a pure SSD.)

My computer at left uses two Samsung 1 TB 970 Pro NVMe M.2 SSD storage devices. Under HFS+, boot time is 19 seconds from pushing Restart to display, although the disk activity light on front panel shows intense activity after that for about 30 seconds. If running under APFS, these Samsung SSDs are very unhappy (due to a TRIM problem) and will take about 59 seconds for the same sequence. I understand that is due to Samsung's controllers. That's only one of the reasons I prefer HFS+ to APFS. There are others.

FWIW, I have no spinning hard drives in any of my four Hacks. My Apple Macintosh PPC G5 uses two WD 1 TB hard drives. But hey, that's an "Early 2005" computer.

 

[mike18xx]

My computer at left uses two Samsung 1 TB 970 Pro NVMe M.2 SSD storage devices. Under HFS+, boot time is 19 seconds from pushing Restart to display, although the disk activity light on front panel shows intense activity after that for about 30 seconds.

Nice. That "intense activity" is probably either Spotlight indexing, or MRT, or both. (I disable both as notorious don't-share-well-with-others disk-access hogs that basically have your drive hulahooping and skipping-rope at the same time at launch (so you really want to avoid it with rotationals). Disabling Spotlight does shut off the built-in Finder search-engine...which isn't big a deal because it sucks anyway compared to the 3rd-party Find-A-File.)

"...If running under APFS, these Samsung SSDs are very unhappy (due to a TRIM problem) and will take about 59 seconds for the same sequence. I understand that is due to Samsung's controllers...."

I wouldn't be so sure that it's Samsung's fault (unless this is a known problem), because APFS has been a sluggish sow on everything I've experienced it run on,* and the more files and more file-churn there's been, the worse it gets. I suppose, in a few years after Apple has deprecated Mojave (and thus HFS+), they'll introduce "APFS+" that solves its performance problems.

(*I have a vanilla 2012 with 1.12TB Fusion drive that will flat out smoke any late-20teens APFS machine in terms of disk-access, which is, subjectively, how most people experience "fast" or "slow" in their computer. Quicker new processors are faster at performing long tasks like a 4k movie render, but I'd wager that 99% of users never put their machines through those kinds of paces. Anything with a Fusion/SSD is basically a Ferrari routinely sent to get groceries in 2nd gear, and it doesn't matter if the top end is 160mph or 250.)

 

[mike18xx]

I finally have an awesome set-up of Mojave in an HFS+ partition that I can infinitely bootable clone/backup with CCC5, and install on 2008-to-'11 machines with Open Core Legacy. Rather than a fresh installation, I straight upgraded an old Mountain Lion archival dmg CCC'd to a spare volume, and wound up getting back my Photoshop CS6 Extended in working order, as well as working Tab Mix Plus extension for Waterfox Classic (for multirow tab grooviness). Runs like greased lightning in HFS+ on an SSD or Fusion-drive intel machine; reboot cycle of twenty seconds, Photoshop launch time two seconds, etc. ...with this, I mostly leave behind High Sierra as my daily driver (although there's really little difference between the two, and none that is noticeable after you ditch the memory-sucking transforming sanddune picture for a static wallpaper. All the life-improving Terminal hacks that work in High Sierra also work in Mojave.