On Mon, 3 Sep 2018 01:06:44 +0800, "Mr. Man-wai Chang"
wrote:

The slow-down is most likely a software issue.

Pardon me to jump to conclusion:

Get/Buy a cheap spare hard disk and clean-install Win 10. See if
everything runs smoothly then. If it really does, you should consider a
full backup of everything and do a clean install with the original hard
disk.
Thanks for the suggestion..... but I have a problem. I have a Win 10
ISO disk, downloaded and saved some time ago, but my Win 10 was an
upgrade from Win 7, and don't have access to my Win 7 product key.
That is, I have it, but it's boxed away in one of dozens of boxes
packed away a year ago after storing everything that wasn't damaged in
the flood of Harvey. Is there some way to read the installed key from
my current installation so I can activate the "new" installation?