I have a 27 inch late 2012 iMac with 32GB of RAM and a 3TB Fusion Drive. The hard drive was replaced in June 2017 so only has a year of wear on it and should thus be in good condition.
For the last couple of months, however, I have been experiencing strange and infuriating performance problems with my machine that are seemingly unexplainable. These problems are most noticeable when using Finder - whenever I try to move or copy more than one folder at a time, Finder will slow down, freeze, beach ball and be rendered all but unusable until the task is complete. The more files I am attempting to copy, the worse this phenomenon is. Upon checking activity monitor I noticed that during these instances, Finder's CPU usage clocks up to 100% + and its memory consumption will instantly balloon from 120MB up to around 1.4GB. Once the process has finished, the CPU usage goes back to normal but the memory consumption does not - yet Finder is noticeably slower.
Finder also has trouble browsing large(ish) folders and is slow to scroll and display new results. This ultimately means that I have to relaunch Finder multiple times a day.
I have done SMC, PRAM, and Safe Boot resets. I have done several clean installs of macOS Sierra and High Sierra. I have removed and tested my RAM modules one by one to no avail. I have taken my iMac to Apple, who tested it for a week and said they found nothing wrong with my machine's hardware and that this was 'normal behaviour' for my machine.
My iMac does occasionally make a soft, croaky/rumbling noice - especially when searching for files. I also noticed that, when I received the computer back from Apple, it was making a quiet ticking noise for a couple of days but this seems to have stopped by itself.
Has anyone encountered problems like this? Are there any further diagnostic or trouble shooting measures which I can undertake?
Kind Regards, James