I have been going nuts recently with Windows 10 being slow - both for customers and on my own machine. Task Manager usually reports 100% disk access. For customers, I was doing what always worked in Windows XP and Windows 7 - stop any unnecessary programs from running at startup and scanning for malware and PUPs (potentially Unwanted Programs). I was largely unsuccessful. Sometimes adding memory helped. For my own machine I was just saying a bunch of bad words.
My computer was so behaving so badly that I decided to look into things a little further. A Google search turned up an answer on a Microsoft forum to a question that said that there is a bug in the firmware of the AHCI (hard drive) controller that causes the controller to incessantly reset. As is typical Microsoft, the answer was for a narrow set of circumstances. It turned out that my computer does not use that particular AHCI controller and the registry key that Microsoft suggested does not exist on my computer.
Further examination of my Google search results turned up the following article from Lifewire:
As suggested in the article,disabling Diagnostics Tracking (Connected User Services and Telemetry) in Services.msc cured the problem. For good measure, I disabled the Windows Search Indexer (Windows Search) in Services.msc. Neither of these services is particularly important to an individual user. Diagnostics Tracking reports diagnostics data to Microsoft, and the Search Indexer appears to spend more time indexing than it saves in search.
I now have a computer that operates properly and a new "trick" to help customers with.