Why my computer is running so slow?

This video describes EXACTLY what has happened to me - except I am on a Mac, not on Windows. I have spent yet more money but to no avail. No one seems to have a handle on this.

Apple Admitted Slowing Down Older iPhones; How Is Your Internet Speed?

http://money.cnn.com/2017/12/21/techn...

https://www.theverge.com/2017/12/28/1...