>>Did you spell it correctly - it's "AllowCortana", case-sensitive (not "AllowCortona").
>
>Yes, AllowCortana is properly written in the Registry.

Have you gone through all your Privacy settings and disabled things that are related to Cortana e.g.

- Speech
- Inking & typing personalization
- Diagnostics & feedback (set to Basic/Never)
- Activity History

and look for Cortana options in all other Privacy sub-pages (there are lots of them).

Out of the box Cortana is enabled and hooks in to a lot of stuff. With the Registry hack and Privacy settings you can stop most of it.

This link explains how to completely stop it (as of the date of the article, may not work now): https://fossbytes.com/how-to-disable-cortana-in-windows-10-working-method/

But it warns that if you use Method 3 then you lose Windows Search, in which case you'd need to install a replacement.

I've found that registry hack + Privacy settings has worked for me, such that I've never noticed Cortana using significant resources.