Chosen Solution
My late 2012 27” i5, without an SSD got very very slow. Disk Read/Write never gets above 20MB/s with normal computer usage until I Run BlackMagic, when disk is capable of hitting over 80MB/s . CPU use is minimal, ram is free and so is disk space. Looks like something is throttling the system disk access, so anything i do is painfully slow. Apple System Diagnostics shows no errors with even the most thoroug scan, surface scan prooved no errors and so did fdisk -fy from single user mode. Heck, what else could I do except trying to run it from an external ssd installed system?
Well I’ve got some bad news ;-{ This macOS version don’t work well on HDD’s as it runs the newer APFS file system and currently there is no means to defragment the HDD like we could with HFS+. It was designed to run on SSD’s which work on a block basis not cylinder/sector a HDD works off of. So to fix this you will need to backup your drive (TimeMachine), then reformat it and install a fresh OS onto it and then restore your user accounts, apps & data (Migration Assistant). The other approach is just bite the bullet and get a SSD swapping out your old spinning rust drive.
