Performance issues won't be fixed by down grading the OS, Often with Hard Disks (spinning rust drive) the issue is file fragmentation. The next issue is Fusion Drive's are not very good if you want performance! They are good for fast boot ups or when you are constantly running the same code over and over. But as the flash cache drive is not very large it doesn't take long to get bogged down.
I would stick with Mojave as I can tell you its not any slower than High Sierra. But, I do agree APFS is not very good on a SATA or mSATA drive your system has. This has to do with the I/O limit of SATA III (6.0 Gb's) and the queue depth of the I/O channel.
Both High Sierra and Mojave use APFS and only if you've upgraded a HD will the older HFS+ file system not get upgraded with High Sierra from Sierra. Which is what the Discord group noted the performance changed after upgrading to Mojave as the installer would in fact update the older HFS+ to APFS at that point no matter the older version!
So that then leads us to jumping back to Sierra to get back to HFS+. But are we just fighting a problem that could be fixed more easily, by upgrading the RAM to 16GB. But thats not possible in the Late 2013 systems as Apple soldered the RAM!
So what to do??
So I would encourage you to make sure your drive is healthy and if it is reformat it with APFS and re-install Mojave first to see if the drives health was the real issue here if you have either a HDD or Fusion Drive.
The next option installing a blade SSD if you only have a HDD system and make that your boot drive. I would go with something in the 512GB size. If you have a Fusion Drive I would still replace the Blade SSD with the 512GB and break the Fusion drive set so the blade drive is your boot drive, that is about all you can do and this would offer much more performance than a SATA HDD!
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@pianowithsam - Before we get into down grading how about telling us a bit more what you are having troubles with.
The reason I ask is Mojave is the better macOS release! Over High Sierra. Frankly, the next older macOS Sierra is better than High Sierra!
High Sierra is the experiment of APFS and you can't back off it without a complete reformat and fresh OS install.
But maybe there is a better answer! So giving us what's troubling you we might have an Ace up our sleeve!
из Dan
Thanks for your comment, @danj
I should clarify that this is my brother’s Mac and it’s a case of wanting to downgrade back to the OS he has been on for a good number of years rather than desiring High Sierra specifically.
He updated to Mojave yesterday and almost instantly noticed reduced performance compared to what he was having on HS. The machine is not recent (late 2013 model purchased in 2015). As well, many people have reported that Discord, the communication platform that he frequently uses, is quite laggy on Mojave. He would rather downgrade back to HS as the compromises of it far outweigh what has been experienced thus far with Mojave.
I’m away that it’s not possible to downgrade without a complete reformat of the hard disk. I just wasn’t sure what the best way was to reinstall the previous OS whilst not losing any data AND keeping in mind that the only Time Machine backup that has been made was on the OS that wants to be downgraded from.
Cheers!
из Sam Mileberg