Finder constantly not responding/freezing
How odd. But I solved it.

Of course, I lost about half my files with the system restores through Time Machine before I worked it out. 

nohillside ♦ pretty much got it spot on. It was me using autofs to mount network shares. Quite why a network share going down or being unavailable means that the entire system is unusable is beyond me. It fact, thinking about it, it's utterly insane.

I wonder if now I have to write some really hacky script to automatically disable autofs if it can't connect to the server a few times in a row.

Another thing that I fear will remain an eternal mystery is why various other apps completely locked up and failed to start. Why TextExpander and BetterTouchTool? Why any file manager? Why Excel but not Word?!

TL;DR - how do I fix it?

Simply deleted the auto_nas file with:

$ sudo nano /etc/auto_nas


I kept the line in /etc/auto_master