Photoshop 25 slow after update from CS6

You don't give us detailed informations about your issue and your system. 

What do you mean with "... and have experienced poor operation on my Windows 10 desktop."?

Please provide a copy of your 'System Info'. This can be get from the Photoshop menu item Help > System Info. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

Is your system up-to-date? Do you have installed all recent updates and patches for the operating system and the device drivers, especially the graphic driver. If you have a NVidia graphic card installed, make sure that you are using the recent Studio version of the driver, not the Game Ready version.

If you still like cancel your plan take a look here:

https://helpx.adobe.com/manage-account/using/cancel-subscription.html

and here:

https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html

If this isn't help, try to enable popups and cookies in your browser or use another browser.

To reinstall you Photoshop CS6 you have to uninstalling all Creative Cloud software:

• uninstall everything cc including preferences, https://helpx.adobe.com/creative-cloud/help/uninstall-remove-app.html
• then uninstall the cc desktop app, https://helpx.adobe.com/creative-cloud/help/uninstall-creative-cloud-desktop-app.html
• clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html
• restart your computer (don't skip this)

After this you can reinstall your CS6. If the online activation failed try to do a offline activation.

https://community.adobe.com/t5/download-install-discussions/signing-into-and-activating-legacy-adobe-applications/td-p/14372541