Despite having Secure Deletion option set to Normal (Faster) it's taking 15-18 minutes to delete the thumbnail cache (about 3Gb in 1,700 files) of an old version Paintshop Pro using Custom File and Folders in Custom Clean, compared to 1-2 minutes in Explorer.
The files aren't being "wiped" as I can still restore them with Recuva, they appear to just be normally deleted, veeerrrrryyyy ssslllloooowwwyyy...
I did notice some increase in activity from Windows Defender during the deletion, which could be a clue, or a red herring
Perhaps it's caused by Windows Defender, and my guess is it's the cloud-based/network-based part of Windows Defender because when I disconnect from the Internet my computer doesn't get any slowdowns whatsoever.
Something Microsoft updated about 4 weeks ago causes some things to take ages even with programs built into Windows such as Disk Cleanup on my system which has more than enough oomph to do things infinitely faster.
After updating another computer to the newest version of Win10 which is 21H2 the slowdown started happening on that computer too, whereas the previous version 21H1 didn't cause that computer any slowdowns.
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Perhaps it is associated with Windows Defender, but not <strong>"cloud-based protection" as I had that disabled</strong>.
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I just disabled Windows Defender's Cloud Delivered Protection, followed by a system restart and now after 4 weeks (going on 5 weeks now) my laptop doesn't have any more lag/slow down anymore. In my case I thought it was caused by Windows Defender because I've changed nothing else on the laptop.
I'd like to thank you for mentioning not having the cloud protection turned on since it gave me a clue of what to disable, and it worked, albeit the system may be more vulnerable now but it's at least usable in an acceptable fashion performance-wise again.