Windows 10 computer is very unresponsive

First I'll describe the problem, then give the background.

The problem is that a Windows 10 computer usually is very unresponsive.  Clicking on anything, the blue circle will spin, then stop.  Then 10-20 seconds later the app will start to come up, but slowly.  The row of icons will appear at a rate of about 3 per second.  Often when I try to type into it, the characters don't appear - until several seconds later, then they will suddenly show up.

I've looked at task manager, and it is showing periods of high CPU use, and it is in these periods when it us unresponsive.  The main things using the CPU are "system", "service host: diagnostic policy service" and "service host: Windows event log".  See the attached screen capture from Task Manager.  When the CPU use is 20% or higher, it is unresponsive.

Background: I got a used HP Z620 last week (two 8-core Xeons, 64GB of RAM).  It came with Windows 7 and was working fine.  I tried to upgrade it to Windows 10, but it got into a boot loop. I took it into the computer repair shop, and they did a reinstall of Windows 10.  But when I got it back, it had this problem.  It would come and go - see the Task Manager image, which shows 4 minutes.  I left it on overnight last night, and this morning the task manager was flat at or near 0%, and it was responding normally.  When I came back after lunch, it had rebooted and the problem was back, except worse.  Now all (or almost all) of the time the CPU use is high and it is unresponsive.  The repair shop guarantees their work, but I'm wondering if there is a quicker solution.