Terrible performance on Mojave with late 2013 iMac - trying (and failing) to downgrade to High Sierra
Recently I have noticed a serious degradation in performance on my late 2013 iMac (running Mojave), to the extent that it is now completely unusable. I get regular spinning wheels, but in Activity Monitor there is no sign of any undue load on CPU or memory consumption. In even the simplest operations the machine just locks up.
Initially I wondered whether there were issues with syncing services (DropBox, iCloud Drive) but disabling them has had no effect.
Next I tried a clean reinstall of Mojave (following a backup to Time Machine), again to no avail, so I wonder whether it is an issue with Mojave itself. Unfortunately I am not able to restore from the Time Machine backup (not a major issue as valuable assets are backed up elsewhere).
So now I am trying to downgrade the installation to High Sierra to see if that fixes the issue. I have managed to create the bootable installer on a USB and have booted the machine from that.
The issue I have is that I cannot install High Sierra as it says the Macintosh HD is locked. After yet more searching I found that to get round that I needed to erase the disk in Disk Utility. I tried that but it sits there for hours doing nothing, stuck at "Deleting Volume". I get the same behaviour as in this post, where it is suggested that APFS is not suited to HDD, however I do not get the option to choose Mac OS Extended (Journaled), only APFS formats.
I have read that it is possible to manually remove the APFS container using Terminal, but before I get too deep into that I wanted to ask anyone's advice on how best to proceed.
Any insights/advice would be much appreciated, before the machine gets thrown out of the window...
iMac 27", macOS 10.14