Posting here to save others the time and hassle I've gone through.
After a year of troubleshooting my 13" HP Spectre x360 with integrated Intel HD Graphics 620 using registry hacks, driver updates and Windows re-installs, I think I have found a work-around for stutters, freezes and lags of the laptop with Windows 10.
Panel Self Refresh does not work and causes what appear to be performance issues, hangs, stutters and lags with Windows 10. No error messages, event logs or other reports.
Right-click on the desktop and open Intel Graphics Settings.
Click on Power
Then for each Plan (select them using the drop down menu) disable Panel Self Refresh.
Ensure that for both Plugged In and Battery (left hand side list) the Plan selected has Panel Self Refresh disabled.
Apply the changes to the settings.
Thanks to some MS Surface Pro 4 users who reported this on Microsoft support pages for the clue.
The issue goes wider than that device.
If you're having lags or stutters then try this and maybe it will work for you.