Mouse tracking speed stuck on slowest setting since 10.12.4 update
I ran into this problem this morning. After trying several things which themselves did not work I finally found the solution:

Uninstall SmoothMouse

The app is no longer supported by the developers and they recommend removing it.


  To Uninstall SmoothMouse  
  
  
  System menu (the Apple logo) -> System Preferences
  Find SmoothMouse in the bottom section and open it
  Open the About tab in the SmoothMouse settings and click Uninstall.
  


You might need to restart afterwards but that fixed it for me.