>>Generally there are 2 parts to questions like this:
>
>I have been unable to disable Cortana a while ago. I see it is still running in the Task Manager.
>
>As far as antivirus, I use the Windows one. Since I moved to Windows 10, I have used the default antivirus. For a long period of time, the performance was optimum. Since a few months, it has started to go down very drastically.

The best way I know to disable Cortana is the registry entry: https://blog.techinline.com/2018/08/07/how-to-disable-cortana-on-windows-10/ . If it's disabled you should have a magnifying glass icon in the task bar instead of the Cortana search box.

Despite being disabled, a Cortana process still runs in Task Manager: https://www.howtogeek.com/271096/why-is-cortana-still-running-in-the-background-after-you-disable-it/

Re your SSD:
- How full is it?
- I'd check that TRIM is enabled in Windows. You could also use the Samsung Magician utility if you're not already; I believe it includes an optimization/manual TRIM option, and can display drive health