Why is My Mac Running So Slowly?
In my experience, unusually slow Macs are usually slow due to hard drive failure, or Malware installation (or in the case of MacBook Pros, a bad hard drive cable). Those are the common causes these days.

Step one: Download SMART Utility and see what it says about your physical drives. If your HDD is failing, it will tell you right away.
(Note: it can't check the SMART status of most external drives, leave your externals disconnected through this whole process, and until the Mac is running normally again)

Step two: Download and run MalwareBytes for Mac, and run it. The free version is all you need. Remove anything it finds. 

If none of these end up being the cause, then you have something much less common going on on your machine.