What devices are connected to your Mac ? And what third party PCIe expansion cards are installed ?
I once had a very similar situation with a Mac Pro 5,1 and a third party USB PCIe Card that had no driver support in macOS . The System was horribly sluggish ( 99 % loss of performance ) , all the CPU threads went to full load and Activity Monitor did not indicate any app was consuming significant resources . It was simply the System attempting to cope with unsatisfied driver requests being made by that expansion card .
Things went back to normal once the card was removed .
Sometimes an external USB Hub can cause an issue like this , too .