If by never runs, you mean things like automatic grammar and spell checking, which way back when were things you'd click a menu to run, and now are constantly running reassessing your lifes worth every time you click a new key. There tend to be any number of 'small' things like this that add up over time and suddenly we're running slow. It also doesn't help many applications followed the Apple school of thought and remove any configuration options so you're unable to disable much of this.

This is ignoring the plethora of bad decisions in the past we're paying for now. Your word processor is likely running in a virtualized environment in your operating system because 30 years ago running full blowin programming applications with no security in your document was a good idea. Then you'll have another layer or two of anti-virus on top of it.