It's a Mac Mini, Late 2012, 2.4 GHz Intel Core i7, 16 GB RAM and an internal fusion drive of 1 TB, running macOS High Sierra 10.13.3.
The issue is that it's running so slowly that there I can move the mouse and it takes up to sixty seconds (on one memorable occasion, eighteen minutes) for the cursor to move on the screen. I launch an application and it's two or three minutes before it's available. I say available, but it's not really available unless I'm prepared to move the mouse and then go for a coffee whilst it acknowledges the action.
Load Avg: 58.82, 38.48, 36.87
It's a relatively new installation - about a month old. There are four USB drives hanging off a powered hub, but disconnecting them does nothing to speed up the system. I launch ActivityMonitor and watch it for a while. The CPU usage is about 98.8% idle. Once I start to use the Mac, I see 'kernel process' which can run up to 1600% of CPU. The fan is on constantly and is extremely loud.
The applications that I can see running (in the Dock as opposed to in the background or via a terminal) are Safari, Mail.app, WhatsApp, iTerm2 (with eight tabs), Gedit and of course, Activity Monitor. Quitting them all doesn't eliminate the 'kernal task', although if I don't touch the Mac, that will disappear after about half an hour.
Here is the output of
top at the moment, when things are relatively calm:
Processes: 289 total, 2 running, 287 sleeping, 1053 threads 15:17:07 Load Avg: 1.56, 2.67, 4.64 CPU usage: 2.78% user, 2.17% sys, 95.3% idle SharedLibs: 316M resident, 63M data, 50M linkedit. MemRegions: 36312 total, 2305M resident, 166M private, 565M shared. PhysMem: 7084M used (1815M wired), 9298M unused. VM: 1277G vsize, 1100M framework vsize, 0(0) swapins, 0(0) swapouts. Networks: packets: 355317/388M in, 229733/35M out. Disks: 390366/11G read, 226405/11G written.
In the menu bar, I previously had stuff such Adobe CC, JumpCut and Skitch. I have killed those, and restarted after removing them from 'startup'. I've also tried the usual stuff like repairing disk permissions etc.
I don't know what to do at this point, and am tearing my hair out in frustration. All advice gratefully received.