Answer to "2013 Mac Pro runs much slower than normal and fan runs at full speed"Run your Activity Monitor. I’ve had the same issue and it turned out to be a single executable, called “kernel_task”which was running away with itself (1,934.6% of CPU), and which I deleted, and the problem was solved, and hasn’t returned. Since then I’ve upgraded my SSD, RAM and CPU — and it’s all still running fine.
Answer to "2013 Mac Pro runs much slower than normal and fan runs at full speed"I have a 2013 MacPro that had the runaway fan problem noted by others. It had been upgraded to a 12-Core 64Gb machine with a 1Tb SSD and an external Thunderbolt-connected 10Tb MaCie Big2 RAID drive, and had been running happily in that configuration for months, until the fan started running at full speed. Typically it would run OK for anywhere from 5 to 20 minutes before the fan would flare up to impressively full speed, with the machine still stone cold, and no sign of overheating anywhere, and performance totally choked. This happened with pretty much any application running, and once with none. I tried re-setting the SMC, VRAM and PRAM, all to no avail. I re-installed Big Sur (don’t get me started on the issues I had with that!) A look at the Activity Monitor revealed that the culprit was somewhere within the kernel library. Processor utilisation was up over 85% and the only sign of abnormal activity was the kernel library. I re-booted in Safe Mode (restart holding down the Shift key) and used Terminal...