There's better ways to approach this. Those folders are locked precisely because people try to do that, then hose their system and blame Apple for it.
Run this analyzer and post the results here:
Etrecheck - http://www.etrecheck.com/
See http://www.etrecheck.com/faq for information about the basic version which is free and how to upgrade this to a power user version.
Using Etrecheck - https://discussions.apple.com/docs/DOC-6174
Extra tips about settings to have it check all parts of your computer, and how to post long reports to the Apple Support Communities window: https://discussions.apple.com/thread/250599211?answerId=251149996022#251149996022
In this era of solid state drives, a full drive should not noticeably slow down a computer because file access is essentially instantaneous. More likely is your Mini with High Sierra is an old machine, possibly with too little RAM, a conventional HDD instead of a SSD. It is probably running the highest macOS it possibly can which is putting a lot of pressure on the machine. Your machine probably hasn't slowed down, it is everything else that has sped up.