Suddenly gone incredibly slow

Win 10 x64 pro
Overnight win10 has become impossibly slow (up to 10 seconds to load a program and often longer). No new programs installed, no viruses and sfc /scannow won't run even in safe mode, which was equally slow. Spent 48 hours trying to deal with this without a complete re-install - repair made no difference. No programs hogging resources. Hardware built for good speed with 4 SSD's. Temps OK and a new install on another disk runs normally. On the point of returning to Win 7 - Win 10 has annoyed me from the start and, frankly, this snail like behaviour comes as no surprise.
Surprising answers would be appreciated

Did you tested the drive for issues with check disk?
If you can boot from the drive that works normally, open a CMD window as administrator and type:
chkdsk x: /f (replace x with the drive appropriate drive letter)
It will say your disk is in use and ask if you want to un mount it = yes

megahertz said:

did you tested the drive for issues with check disk?
If you can boot from the drive that works normally, open a cmd window as administrator and type:
Chkdsk x: /f (replace x with the drive appropriate drive letter)
it will say your disk is in use and ask if you want to un mount it = yes

100% ok

- - - Updated - - -

Further to this problem I am trying to perform a re-install keeping settings etc but get the dreaded "This upgrade path is not supported. Please close Setup and re-launch from the root of the media or go back and pick a different installation choice."
Same using root and adding my Key.
Is there an answer to this?

Try a Repair Install Windows 10 with an In-place Upgrade

Megahertz said:

Try a Repair Install Windows 10 with an In-place Upgrade

Just like that! It doesn't work for me and I'm going witless repeating things over and over again. It's taken me months to tame Win 10 and install dozens of programs so a re-install is not on.
I just tried to load up Win 7x 64 which I thought I understood but it won't boot.
Sorry for the rave but I've never had this sort of trouble in 35 years. MS is now worse than Apple imho.

A Repair Install Windows 10 with an In-place Upgrade reinstall windows and keep your data, settings and programs and it won't take more then 1/2 a hour.

To install win 7 you need to add some drivers to the installation files.
Installation & Setup - Windows 7 Forums

Megahertz said:

A Repair Install Windows 10 with an In-place Upgrade reinstall windows and keep your data, settings and programs and it won't take more then 1/2 a hour.

To install win 7 you need to add some drivers to the installation files.
Installation & Setup - Windows 7 Forums

Once I'd installed the update to 21h1 the 'repair' took over 3 hours but I ended up with what I wanted. All went well for about 30 mins and suddenly stopped with a problem and a reboot. I then found myself with all my personal data scrambled.
e.g. Firefox and Thunderbird needing re-install. Oddly my Acronis backups on another disk had been porked as well which is very odd.
Been here all day repairing links etc. Restoring Windows.old failed although it was there on the disk and copying them over failed to restore my user profile which I put down to MS paranoia
Repair failed miserably from the very same USB image that got me here in the first place.
It's a nightmare that this 80 year old is struggling to handle. Can anyone suggest a reason that MS porked it's own recovery?

New user entered but what was the point of it?